Payment method is automatic:
How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359
"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 ID balance to cover the full amount.
2. If you don't have Apple ID 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 may try again."
Apple may require you enter a payment card for "verification" purposes (they no do not say what they mean by that) but will not take money from the card if you have enough Apple ID balance. You can try removing the card one you have verified.