Execute a multi-item Steam microtransaction purchase using ISteamMicroTxn InitTxn

Steps

1. Gather the list of items to purchase and map each to its itemid, description, quantity, and amount (in the currency's smallest unit)
2. Call the ISteamMicroTxn/InitTxn/v3 endpoint using indexed parameters: itemid[0], qty[0], amount[0], description[0] for the first item, itemid[1], qty[1], amount[1], description[1] for the second, and so on
3. Pass the required top-level parameters including the user's Steam ID, the app ID, the language, and the currency code
4. Parse the returned transaction ID (orderid) and steam transaction handle from the InitTxn response
5. Redirect the user to the Steam payment overlay or page using the returned transaction handle to complete authorization
6. After the user authorizes, call ISteamMicroTxn/FinalizeTxn/v2 with the orderid to capture and complete the transaction