Carrier billing lets you buy apps, games, or services and charge the cost...

https://blogfreely.net/neasalnfbo/carrier-billing-stopped-working-after-i-changed-my-plan-what-you-need-to-know

Carrier billing lets you buy apps, games, or services and charge the cost directly to your monthly phone bill. If you use a prepaid plan, the amount is taken from your available pay-as-you-go credit