To implement in-app purchases on Android, under the hood we use Google Play Billing Library. We have just upgraded the way we use it, to follow the latest API (no AIDL, and use built-in asynchronous methods).
This is important if you use in-app purchases on Android. Since November 2021 (in one month) Google will no longer allow uploading Android builds (APK, AAB) that use the old API.
There’s no change to the Castle Game Engine public API. Just use the Pascal unit CastleInAppPurchases, following the documentation how to implement in-app purchases on both Android and iOS. Both consumable and non-consumable in-app purchases are supported.
The Unholy Society is already using the new API for our only purchase in the game.