If you are looking to make Android in-app purchases outside of the Google Play Store, you may want to check out some third-party options. As for customizing the GodotGooglePlayBilling library, it is always worth exploring the documentation and community forums to see if others have tried similar modifications. You may want to also consider achieving out to the Godot development team or neighborhood for guidance on making the changes you are looking for. Best of luck with your in-app purchases and customizations!