Greetings PSUMLN,
Here is some information on how Apple bills purchases - How apps, content, and subscriptions from Apple are billed
Which payment method is charged?
When you buy items or pay for subscriptions, Apple attempts to charge your payment methods in this order:
1. Apple attempts to use your Apple Account balance to cover the full amount.
2. If you don't have Apple Account balance or you don't have enough to cover the full amount, Apple attempts to charge your primary payment method. The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.
3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple might try again.
Some purchases aren’t charged to your Apple Account balance
• If you're in a Family Sharing group, purchases that you make are charged to your personal Apple Account balance. If you don't have enough Apple Account balance to pay for the purchase, the remainder is charged to the family organizer if purchase sharing is turned on.
• You can’t use your Apple Account balance to buy gifts or gift cards.
• Some subscriptions might not be charged to your Apple Account balance.
Should this not be what's happening, to further explore the situation you'll want to reach out to Apple Support and work them more closely on the matter. Here's a link to do that - Contact Apple Support
Thanks.