Implement Steam microtransactions using the ISteamMicroTxn web API for in-game purchases

Steps

1. On the server, call the InitTxn endpoint of the ISteamMicroTxn web API with the player's Steam ID, your app ID, a unique order ID, the list of items with amounts and descriptions, and your publisher web API key.
2. Receive the transaction ID (txnid) from the InitTxn response and pass it to the game client.
3. On the client, call ISteamMicroTxn::InitiateGamePurchase (or the equivalent overlay trigger) with the txnid to display the Steam purchase dialog to the player.
4. Listen for the MicroTxnAuthorizationResponse_t callback on the client; on approval, notify your server with the order ID.
5. On the server, call the FinalizeTxn endpoint with the order ID and txnid to complete the transaction and charge the player; only grant items after this call succeeds.
6. Handle edge cases: check QueryTxn to verify transaction status before granting items if your server did not receive the finalization response cleanly.