Handle server-to-server purchase verification for in-game items across Apple, Google, and Steam from a unified backend

Steps

1. Define a unified receipt payload your game client sends to your backend: platform identifier (apple/google/steam), purchaseToken or transactionId, productId, and player ID
2. Route to the platform-specific validation logic: for Apple call the App Store Server API GET /inApps/v1/transactions/{transactionId}; for Google call androidpublisher purchases.products.get; for Steam call ISteamMicroTxn/QueryTxn after FinalizeTxn
3. On validation success, write an idempotent grant record keyed by (platform, transactionId/token) before granting the item — check for duplicates first to prevent replay attacks
4. Grant the item via your server-side inventory service (e.g. PlayFab AddInventoryItems or your own database) only after the duplicate check passes
5. Return a structured response to the client indicating success, already-claimed, or validation-failure; never reveal raw platform error messages to the client