diff --git a/docs/api-reference.md b/docs/api-reference.md
index 142ff18..ff55c46 100644
--- a/docs/api-reference.md
+++ b/docs/api-reference.md
@@ -457,22 +457,93 @@
 ```json
 {
   "success": true,
-  "data": [
-    {
-      "id": "restore-hearts",
-      "type": "hearts",
-      "title": "恢复满血",
-      "description": "血量不足时继续挑战",
-      "enabled": true,
-      "requiresAd": true
-    }
-  ],
+  "data": {
+    "benefits": [
+      {
+        "id": "restore-hearts",
+        "type": "hearts",
+        "title": "恢复满血",
+        "description": "血量不足时继续挑战",
+        "enabled": true,
+        "requiresAd": true
+      }
+    ],
+    "products": [
+      {
+        "id": "hint-feather",
+        "type": "item",
+        "itemId": "hint_feather",
+        "title": "提示羽毛",
+        "description": "答题时排除 1 个错误选项",
+        "priceCoins": 80,
+        "quantity": 1,
+        "enabled": true
+      }
+    ]
+  },
   "error": null
 }
 ```
 
 说明：`requiresAd=true` 的权益应通过 `/rewards/ad-recovery/session` 和 `/rewards/ad-recovery/complete` 完成资格检查和结算。
 
+可购买商品价格：提示羽毛 80 金币，爱心补给 150 金币，双倍 XP 药水 250 金币，连胜护盾 400 金币，第一版吉祥物装扮 800 金币。
+
+#### POST /shop/purchase
+
+认证：JWT
+
+请求：
+
+```json
+{
+  "productId": "hint-feather",
+  "clientRequestId": "request-uuid"
+}
+```
+
+`productId` 取值：`hint-feather`, `heart-supply`, `double-xp-potion`, `streak-shield`, `mascot-outfit-starter`。
+
+响应：
+
+```json
+{
+  "success": true,
+  "data": {
+    "product": {
+      "id": "hint-feather",
+      "type": "item",
+      "itemId": "hint_feather",
+      "title": "提示羽毛",
+      "description": "答题时排除 1 个错误选项",
+      "priceCoins": 80,
+      "quantity": 1,
+      "enabled": true
+    },
+    "coinsSpent": 80,
+    "coinsBalance": 220,
+    "item": {
+      "itemId": "hint_feather",
+      "quantity": 3,
+      "activeUntil": null,
+      "metadata": null
+    },
+    "rewards": [
+      {
+        "type": "item",
+        "source": "inventory",
+        "itemId": "hint_feather",
+        "quantity": 1,
+        "title": "提示羽毛 x1"
+      }
+    ]
+  },
+  "error": null
+}
+```
+
+购买使用 `clientRequestId` 作为幂等边界；金币不足时返回统一错误格式，`error.code` 为 `VALIDATION_ERROR`。
+
 #### GET /subscription
 
 认证：JWT
diff --git a/docs/gamification-server-plan.md b/docs/gamification-server-plan.md
index 426bcd2..89aa186 100644
--- a/docs/gamification-server-plan.md
+++ b/docs/gamification-server-plan.md
@@ -75,7 +75,7 @@
 | G3-1 | 实现金币发放服务 | [x] | 支持每日首组挑战 20、每日任务 30-80、升级 100、主题节点 50、宝箱 20-200 |
 | G3-2 | 实现宝箱奖励服务 | [x] | 支持基础概率、10 连对概率提升、高奖励次数耗尽后的概率降级 |
 | G3-3 | 实现道具库存服务 | [x] | 支持连胜护盾、双倍 XP 药水、爱心补给、提示羽毛的获得和消耗 |
-| G3-4 | 实现商店商品和购买接口 | [ ] | 商品价格符合设计：提示羽毛 80、爱心补给 150、双倍 XP 250、连胜护盾 400、装扮 800-3000 |
+| G3-4 | 实现商店商品和购买接口 | [x] | 商品价格符合设计：提示羽毛 80、爱心补给 150、双倍 XP 250、连胜护盾 400、装扮 800-3000 |
 | G3-5 | 实现道具使用接口 | [ ] | 爱心补给恢复满心，双倍 XP 药水 15 分钟生效，提示羽毛返回可排除选项，连胜护盾可保护断签 |
 | G3-6 | 更新 bootstrap/shop DTO | [ ] | 客户端能拿到金币、库存、可购买商品、广告商品、订阅权益 |
 | G3-7 | 添加金币/商店测试 | [ ] | 覆盖余额不足、重复购买、使用道具、药水时效、库存扣减和流水记录 |
@@ -83,6 +83,7 @@
 验证记录（2026-05-13）：G3-1 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/gamification/coin-service.test.ts src/__tests__/services/learning/challenge-service.test.ts` 仍在启动阶段被 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
 验证记录（2026-05-13）：G3-2 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/gamification/chest-service.test.ts src/__tests__/services/gamification/coin-service.test.ts src/__tests__/services/gamification-rules.test.ts` 仍在启动阶段被同一个 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
 验证记录（2026-05-13）：G3-3 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/gamification/inventory-service.test.ts` 仍在启动阶段被同一个 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
+验证记录（2026-05-13）：G3-4 已通过 `./node_modules/.bin/tsc --noEmit`、`./node_modules/.bin/eslint .` 和 `git diff --check`；定向运行 `./node_modules/.bin/vitest run src/__tests__/services/shop/shop-service.test.ts src/__tests__/services/gamification/coin-service.test.ts src/__tests__/services/gamification/inventory-service.test.ts` 仍在启动阶段被同一个 `@rolldown/binding-darwin-x64` 原生 binding 未签名问题阻塞。
 
 ## Phase G4：广告恢复与订阅权益对齐
 
diff --git a/src/__tests__/services/gamification/coin-service.test.ts b/src/__tests__/services/gamification/coin-service.test.ts
index b72a8b1..d440bbd 100644
--- a/src/__tests__/services/gamification/coin-service.test.ts
+++ b/src/__tests__/services/gamification/coin-service.test.ts
@@ -1,6 +1,6 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { db } from '../../../db/client.js';
-import { createCoinReward, getCoinRewardAmount, grantCoins } from '../../../services/gamification/coin-service.js';
+import { createCoinReward, getCoinRewardAmount, grantCoins, spendCoins } from '../../../services/gamification/coin-service.js';
 
 function selectRows(rows: unknown[]) {
   return {
@@ -130,4 +130,82 @@ describe('coin-service', () => {
     expect(db.insert).not.toHaveBeenCalled();
     expect(db.update).not.toHaveBeenCalled();
   });
+
+  it('spends coins with a consume transaction for shop purchases', async () => {
+    const insertValues = vi.fn();
+    const updateSet = vi.fn();
+    mockSelectQueue([
+      [], // no existing spend transaction
+      [{ coinsBalance: 300 }],
+    ]);
+    vi.mocked(db.insert).mockReturnValue(mockInsert(insertValues));
+    vi.mocked(db.update).mockReturnValue(mockUpdate(updateSet));
+
+    const result = await spendCoins({
+      userId: 'user-1',
+      amount: 80,
+      sourceId: 'shop:request-1',
+      idempotencyKey: 'shop:request-1:coins',
+      snapshot: { productId: 'hint-feather' },
+    });
+
+    expect(result).toEqual({
+      spent: 80,
+      applied: true,
+      balanceBefore: 300,
+      balanceAfter: 220,
+    });
+    expect(updateSet).toHaveBeenCalledWith(expect.objectContaining({
+      coinsBalance: expect.any(Object),
+      lifetimeCoinsSpent: expect.any(Object),
+    }));
+    expect(insertValues).toHaveBeenCalledWith(expect.objectContaining({
+      userId: 'user-1',
+      itemId: 'coins',
+      direction: 'consume',
+      quantityDelta: -80,
+      balanceAfter: 220,
+      sourceType: 'shop_purchase',
+      sourceId: 'shop:request-1',
+      idempotencyKey: 'shop:request-1:coins',
+    }));
+  });
+
+  it('does not spend coins twice for a duplicate idempotency key', async () => {
+    mockSelectQueue([
+      [{ id: 'tx-1' }],
+      [{ coinsBalance: 220 }],
+    ]);
+
+    const result = await spendCoins({
+      userId: 'user-1',
+      amount: 80,
+      sourceId: 'shop:request-1',
+      idempotencyKey: 'shop:request-1:coins',
+    });
+
+    expect(result).toEqual({
+      spent: 80,
+      applied: false,
+      balanceBefore: 220,
+      balanceAfter: 220,
+    });
+    expect(db.insert).not.toHaveBeenCalled();
+    expect(db.update).not.toHaveBeenCalled();
+  });
+
+  it('throws when spending more coins than the current balance', async () => {
+    mockSelectQueue([
+      [],
+      [{ coinsBalance: 20 }],
+    ]);
+
+    await expect(
+      spendCoins({
+        userId: 'user-1',
+        amount: 80,
+        sourceId: 'shop:request-1',
+      }),
+    ).rejects.toThrow('金币余额不足');
+  });
 });
diff --git a/src/__tests__/services/shop/shop-service.test.ts b/src/__tests__/services/shop/shop-service.test.ts
index b90b558..5ea0d71 100644
--- a/src/__tests__/services/shop/shop-service.test.ts
+++ b/src/__tests__/services/shop/shop-service.test.ts
@@ -1,7 +1,39 @@
-import { describe, expect, it } from 'vitest';
-import { getShopBenefits } from '../../../services/shop/shop-service.js';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { db } from '../../../db/client.js';
+import { getShopBenefits, getShopCatalog, purchaseShopProduct } from '../../../services/shop/shop-service.js';
+
+function selectRows(rows: unknown[]) {
+  return {
+    from: vi.fn().mockReturnValue({
+      where: vi.fn().mockReturnValue({
+        limit: vi.fn().mockResolvedValue(rows),
+      }),
+    }),
+  };
+}
+
+function mockSelectQueue(queue: unknown[][]) {
+  let index = 0;
+  vi.mocked(db.select).mockImplementation((() => {
+    const rows = index < queue.length ? queue[index]! : [];
+    index += 1;
+    return selectRows(rows);
+  }) as never);
+}
+
+function mockInsert(valuesSpy: ReturnType<typeof vi.fn>) {
+  return { values: valuesSpy.mockResolvedValue(undefined) } as never;
+}
+
+function mockUpdate(setSpy: ReturnType<typeof vi.fn>) {
+  return { set: setSpy.mockReturnValue({ where: vi.fn().mockResolvedValue(undefined) }) } as never;
+}
 
 describe('shop-service', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
   it('returns the client shop benefit catalog', async () => {
     const benefits = await getShopBenefits();
 
@@ -13,4 +45,75 @@ describe('shop-service', () => {
     ]);
     expect(benefits.every((item) => item.enabled)).toBe(true);
   });
+
+  it('returns shop products with configured prices', async () => {
+    const catalog = await getShopCatalog();
+
+    expect(catalog.benefits).toHaveLength(4);
+    expect(catalog.products.map((item) => [item.id, item.priceCoins])).toEqual([
+      ['hint-feather', 80],
+      ['heart-supply', 150],
+      ['double-xp-potion', 250],
+      ['streak-shield', 400],
+      ['mascot-outfit-starter', 800],
+    ]);
+  });
+
+  it('purchases a product by spending coins and granting inventory', async () => {
+    const insertValues = vi.fn();
+    const updateSet = vi.fn();
+    mockSelectQueue([
+      [], // spendCoins: no existing spend
+      [{ coinsBalance: 300 }],
+      [], // grantInventoryItem: no existing item grant
+      [], // current item
+      [], // no existing inventory row
+      [{ id: 'inventory-1' }], // inventory row id for transaction
+    ]);
+    vi.mocked(db.insert).mockReturnValue(mockInsert(insertValues));
+    vi.mocked(db.update).mockReturnValue(mockUpdate(updateSet));
+
+    const result = await purchaseShopProduct('user-1', 'hint-feather', 'request-1');
+
+    expect(result.product.id).toBe('hint-feather');
+    expect(result.coinsSpent).toBe(80);
+    expect(result.coinsBalance).toBe(220);
+    expect(result.item).toEqual({
+      itemId: 'hint_feather',
+      quantity: 1,
+      activeUntil: null,
+      metadata: null,
+    });
+    expect(result.rewards).toEqual([
+      expect.objectContaining({ type: 'item', itemId: 'hint_feather', quantity: 1 }),
+    ]);
+    expect(updateSet).toHaveBeenCalledWith(expect.objectContaining({
+      coinsBalance: expect.any(Object),
+      lifetimeCoinsSpent: expect.any(Object),
+    }));
+    expect(insertValues).toHaveBeenCalledWith(expect.objectContaining({
+      itemId: 'coins',
+      direction: 'consume',
+      quantityDelta: -80,
+      sourceType: 'shop_purchase',
+      idempotencyKey: 'shop:request-1:coins',
+    }));
+    expect(insertValues).toHaveBeenCalledWith(expect.objectContaining({
+      itemId: 'hint_feather',
+      direction: 'grant',
+      sourceType: 'shop_purchase',
+      idempotencyKey: 'shop:request-1:item',
+    }));
+  });
+
+  it('throws when the user does not have enough coins', async () => {
+    mockSelectQueue([
+      [], // no existing spend
+      [{ coinsBalance: 20 }],
+    ]);
+
+    await expect(
+      purchaseShopProduct('user-1', 'hint-feather', 'request-1'),
+    ).rejects.toThrow('金币余额不足');
+  });
 });
diff --git a/src/routes/app-api.ts b/src/routes/app-api.ts
index 8ad3266..8476208 100644
--- a/src/routes/app-api.ts
+++ b/src/routes/app-api.ts
@@ -12,7 +12,7 @@ import {
 } from '../services/learning/progress-summary-service.js';
 import { restoreHearts } from '../services/progress/progress-service.js';
 import { getClientLeaderboard, getClientLeaderboardMe } from '../services/learning/leaderboard-api-service.js';
-import { getShopBenefits } from '../services/shop/shop-service.js';
+import { getShopCatalog, purchaseShopProduct } from '../services/shop/shop-service.js';
 import { getClientSubscription, verifyClientSubscription } from '../services/subscription/subscription-api-service.js';
 
 const rewardSourceSchema = z.object({
@@ -46,6 +46,11 @@ const subscriptionVerifySchema = z.object({
   tier: z.enum(['pro', 'proplus']),
 });
 
+const shopPurchaseSchema = z.object({
+  productId: z.enum(['hint-feather', 'heart-supply', 'double-xp-potion', 'streak-shield', 'mascot-outfit-starter']),
+  clientRequestId: z.string().min(1).max(80),
+});
+
 function getUserId(request: { user: unknown }): string {
   return (request.user as { userId: string }).userId;
 }
@@ -152,7 +157,14 @@ export async function appApiRoutes(app: FastifyInstance): Promise<void> {
   });
 
   app.get('/shop', async () => {
-    const data = await getShopBenefits();
+    const data = await getShopCatalog();
+    return { success: true, data, error: null };
+  });
+
+  app.post('/shop/purchase', async (request) => {
+    const parsed = shopPurchaseSchema.safeParse(request.body);
+    if (!parsed.success) return validationError(parsed.error.issues[0]?.message);
+    const data = await purchaseShopProduct(getUserId(request), parsed.data.productId, parsed.data.clientRequestId);
     return { success: true, data, error: null };
   });
 
diff --git a/src/services/gamification/coin-service.ts b/src/services/gamification/coin-service.ts
index cd96cfb..33c43bb 100644
--- a/src/services/gamification/coin-service.ts
+++ b/src/services/gamification/coin-service.ts
@@ -2,6 +2,7 @@ import { db } from '../../db/client.js';
 import { inventoryTransactions, rewardLedger, userDailyProgress, userWallets } from '../../db/schema.js';
 import { and, eq, sql } from 'drizzle-orm';
 import { v4 as uuid } from 'uuid';
+import { ValidationError } from '../../utils/errors.js';
 import { COIN_RULES } from './rules.js';
 
 export type CoinRewardSource =
@@ -37,6 +38,21 @@ export interface GrantCoinsResult {
   balanceAfter: number;
 }
 
+export interface SpendCoinsInput {
+  userId: string;
+  amount: number;
+  sourceId: string;
+  idempotencyKey?: string;
+  snapshot?: Record<string, unknown>;
+}
+
+export interface SpendCoinsResult {
+  spent: number;
+  applied: boolean;
+  balanceBefore: number;
+  balanceAfter: number;
+}
+
 const COIN_REWARD_TITLES: Readonly<Record<CoinRewardSource, string>> = Object.freeze({
   first_daily_challenge: '每日首组挑战',
   daily_task: '每日任务',
@@ -189,6 +205,64 @@ async function getCoinBalance(userId: string): Promise<number> {
   return wallet?.coinsBalance ?? 0;
 }
 
+export async function spendCoins(input: SpendCoinsInput): Promise<SpendCoinsResult> {
+  const amount = normalizeSpendAmount(input.amount);
+  const idempotencyKey = input.idempotencyKey ?? `shop_purchase:${input.sourceId}:coins`;
+
+  const [existing] = await db
+    .select({ id: inventoryTransactions.id })
+    .from(inventoryTransactions)
+    .where(and(
+      eq(inventoryTransactions.userId, input.userId),
+      eq(inventoryTransactions.idempotencyKey, idempotencyKey),
+    ))
+    .limit(1);
+
+  const balanceBefore = await getCoinBalance(input.userId);
+  if (existing) {
+    return {
+      spent: amount,
+      applied: false,
+      balanceBefore,
+      balanceAfter: balanceBefore,
+    };
+  }
+
+  if (balanceBefore < amount) {
+    throw new ValidationError('金币余额不足');
+  }
+
+  const balanceAfter = balanceBefore - amount;
+  await db
+    .update(userWallets)
+    .set({
+      coinsBalance: sql`GREATEST(COALESCE(coins_balance, 0) - ${amount}, 0)`,
+      lifetimeCoinsSpent: sql`COALESCE(lifetime_coins_spent, 0) + ${amount}`,
+    })
+    .where(eq(userWallets.userId, input.userId));
+
+  await db.insert(inventoryTransactions).values({
+    id: uuid(),
+    userId: input.userId,
+    itemId: 'coins',
+    direction: 'consume',
+    quantityDelta: -amount,
+    balanceAfter,
+    sourceType: 'shop_purchase',
+    sourceId: input.sourceId,
+    idempotencyKey,
+    snapshot: {
+      source: 'shop_purchase',
+      sourceId: input.sourceId,
+      coinsBalanceBefore: balanceBefore,
+      coinsBalanceAfter: balanceAfter,
+      ...(input.snapshot ?? {}),
+    },
+  });
+
+  return { spent: amount, applied: true, balanceBefore, balanceAfter };
+}
+
 async function upsertWalletForGrant(userId: string, amount: number, balanceBefore: number): Promise<void> {
   if (balanceBefore === 0) {
     const [wallet] = await db
@@ -249,3 +323,10 @@ function clampCoins(value: number | undefined, min: number, max: number): number
   if (typeof value !== 'number' || Number.isNaN(value)) return min;
   return Math.max(min, Math.min(max, Math.floor(value)));
 }
+
+function normalizeSpendAmount(value: number): number {
+  if (!Number.isFinite(value) || value <= 0) {
+    throw new ValidationError('金币消耗数量必须大于 0');
+  }
+  return Math.floor(value);
+}
diff --git a/src/services/shop/shop-service.ts b/src/services/shop/shop-service.ts
index 5670b7f..0a4d3b7 100644
--- a/src/services/shop/shop-service.ts
+++ b/src/services/shop/shop-service.ts
@@ -1,4 +1,8 @@
-import type { ShopBenefitDto } from '../../types/app-api.js';
+import { ValidationError } from '../../utils/errors.js';
+import { spendCoins } from '../gamification/coin-service.js';
+import { createInventoryReward, grantInventoryItem } from '../gamification/inventory-service.js';
+import { ITEM_RULES, type InventoryItemId } from '../gamification/rules.js';
+import type { ShopBenefitDto, ShopCatalogDto, ShopProductDto, ShopProductId, ShopPurchaseResultDto } from '../../types/app-api.js';
 
 const SHOP_BENEFITS: readonly ShopBenefitDto[] = Object.freeze([
   {
@@ -35,6 +39,121 @@ const SHOP_BENEFITS: readonly ShopBenefitDto[] = Object.freeze([
   },
 ]);
 
+const SHOP_PRODUCTS: readonly ShopProductDto[] = Object.freeze([
+  {
+    id: 'hint-feather',
+    type: 'item',
+    itemId: ITEM_RULES.hintFeather.id,
+    title: '提示羽毛',
+    description: '答题时排除 1 个错误选项',
+    priceCoins: ITEM_RULES.hintFeather.shopPriceCoins,
+    quantity: 1,
+    enabled: true,
+  },
+  {
+    id: 'heart-supply',
+    type: 'item',
+    itemId: ITEM_RULES.heartSupply.id,
+    title: '爱心补给',
+    description: '使用后恢复满爱心',
+    priceCoins: ITEM_RULES.heartSupply.shopPriceCoins,
+    quantity: 1,
+    enabled: true,
+  },
+  {
+    id: 'double-xp-potion',
+    type: 'item',
+    itemId: ITEM_RULES.doubleXpPotion.id,
+    title: '双倍 XP 药水',
+    description: '使用后 15 分钟内 XP 翻倍',
+    priceCoins: ITEM_RULES.doubleXpPotion.shopPriceCoins,
+    quantity: 1,
+    enabled: true,
+  },
+  {
+    id: 'streak-shield',
+    type: 'item',
+    itemId: ITEM_RULES.streakShield.id,
+    title: '连胜护盾',
+    description: '忙碌时保护 1 天连续学习',
+    priceCoins: ITEM_RULES.streakShield.shopPriceCoins,
+    quantity: 1,
+    enabled: true,
+  },
+  {
+    id: 'mascot-outfit-starter',
+    type: 'cosmetic',
+    itemId: ITEM_RULES.mascotOutfit.id,
+    title: '多奇出行装',
+    description: '第一版吉祥物装扮',
+    priceCoins: ITEM_RULES.mascotOutfit.shopPriceCoinsMin,
+    quantity: 1,
+    enabled: true,
+  },
+]);
+
 export async function getShopBenefits(): Promise<readonly ShopBenefitDto[]> {
   return SHOP_BENEFITS;
 }
+
+export async function getShopCatalog(): Promise<ShopCatalogDto> {
+  return {
+    benefits: SHOP_BENEFITS,
+    products: SHOP_PRODUCTS,
+  };
+}
+
+export async function purchaseShopProduct(
+  userId: string,
+  productId: ShopProductId,
+  clientRequestId: string,
+): Promise<ShopPurchaseResultDto> {
+  const product = getShopProduct(productId);
+  if (!product.enabled) {
+    throw new ValidationError('商品暂不可购买');
+  }
+
+  const sourceId = `shop:${clientRequestId}`;
+  const spend = await spendCoins({
+    userId,
+    amount: product.priceCoins,
+    sourceId,
+    idempotencyKey: `${sourceId}:coins`,
+    snapshot: { productId: product.id },
+  });
+  const inventory = await grantInventoryItem({
+    userId,
+    itemId: product.itemId as InventoryItemId,
+    quantity: product.quantity,
+    sourceType: 'shop_purchase',
+    sourceId,
+    idempotencyKey: `${sourceId}:item`,
+    metadata: product.type === 'cosmetic' ? { productId: product.id, title: product.title } : undefined,
+    snapshot: {
+      productId: product.id,
+      priceCoins: product.priceCoins,
+      coinsSpentApplied: spend.applied,
+    },
+  });
+
+  return {
+    product,
+    coinsSpent: spend.applied ? product.priceCoins : 0,
+    coinsBalance: spend.balanceAfter,
+    item: {
+      itemId: inventory.item.itemId,
+      quantity: inventory.item.quantity,
+      activeUntil: inventory.item.activeUntil?.toISOString() ?? null,
+      metadata: inventory.item.metadata,
+    },
+    rewards: [createInventoryReward(product.itemId as InventoryItemId, product.quantity)],
+  };
+}
+
+function getShopProduct(productId: ShopProductId): ShopProductDto {
+  const product = SHOP_PRODUCTS.find((item) => item.id === productId);
+  if (!product) {
+    throw new ValidationError('商品不存在');
+  }
+  return product;
+}
diff --git a/src/types/app-api.ts b/src/types/app-api.ts
index b6ea872..781f0f0 100644
--- a/src/types/app-api.ts
+++ b/src/types/app-api.ts
@@ -64,6 +64,50 @@ export interface ShopBenefitDto {
   requiresAd: boolean;
 }
 
+export type ShopProductType = 'item' | 'cosmetic';
+export type ShopProductId =
+  | 'hint-feather'
+  | 'heart-supply'
+  | 'double-xp-potion'
+  | 'streak-shield'
+  | 'mascot-outfit-starter';
+
+export interface ShopProductDto {
+  id: ShopProductId;
+  type: ShopProductType;
+  itemId: string;
+  title: string;
+  description: string;
+  priceCoins: number;
+  quantity: number;
+  enabled: boolean;
+}
+
+export interface ShopCatalogDto {
+  benefits: readonly ShopBenefitDto[];
+  products: readonly ShopProductDto[];
+}
+
+export interface ShopPurchaseResultDto {
+  product: ShopProductDto;
+  coinsSpent: number;
+  coinsBalance: number;
+  item: {
+    itemId: string;
+    quantity: number;
+    activeUntil: string | null;
+    metadata: Record<string, unknown> | null;
+  };
+  rewards: ReadonlyArray<{
+    type: string;
+    source?: string;
+    amount?: number;
+    itemId?: string;
+    quantity?: number;
+    title?: string;
+  }>;
+}
+
 export interface BootstrapDto {
   user: UserBriefDto;
   progress: ProgressSummaryDto;