Actually you do not get to select your payment method, it is done automatically. What it probably wants is for you to enter a credit or debit card for "verification" purposes (don't ask me what that means -- I have spent years wondering). Then it will proceed to charge according to:
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.
Create or use your Apple ID without a payment method - https://support.apple.com/HT204034
- "You can remove payment methods from your existing Apple ID after you sign in to the App Store, iTunes Store, or Apple Books. You won't be asked for a payment method again until you make a purchase."