Carrier billing lets you buy apps, games, or services and charge them directly...

https://echo-wiki.win/index.php/Can_I_Turn_Off_Carrier_Billing_on_My_Phone_Plan%3F

Carrier billing lets you buy apps, games, or services and charge them directly to your phone bill. If you have a monthly plan, the cost appears on your next bill. For pay-as-you-go users, the amount is taken from your available credit