Google Play Billing 7.0.0 for Godot 4+
1 - Add godot_google_play_billing folder to your project's addons folder.
2 - Go to Project/Project Settings/Plugins and enable the plugin.
3 - Open billing.gd file.
4 - Populate const CONSUMABLE_ITEMS: PackedStringArray, const NON_CONSUMABLE_ITEMS: PackedStringArray and const SUBSCRIPTION_ITEMS: PackedStringArray for your own use.
5 - Add reward functionality under the _handle_purchase(purchase_token: String) function.
6 - Calling Billing.purchase(product_id: String) anywhere will initiate the process.