How do you temporarily change credit card use to make a one time purchase?

Hi, 8fE15-gG3.

To double-check, are you asking how to add a new payment method for your Apple ID or change the order of payments? If so, this should help to Change, add, or remove Apple ID payment methods:

"How to change your payment method on your iPhone, iPad, or iPod touch

1. Open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping. You might be asked to sign in with your Apple ID.
4. From here, you can do these things:

• 
• Add a payment method — Tap Add Payment Method.
• Update a payment method — Tap the payment method, then edit your information. You might need to enter the security code that's printed on your credit or debit card. If you use the card with Apple Pay, go to the Wallet app to edit the card information.
• Remove a payment method — Tap Edit, tap the red Delete button, then tap Remove. If you have an unpaid balance, have subscriptions including an iCloud storage plan, or you use purchase sharing, you can't remove all payment methods. Learn how to remove all payment methods.
• Change the order of your payment methods — Tap Edit, then drag the move order icon ."

You can use the steps above to re-order your payment methods, or add a new one. If this doesn't cover your question, feel free to reply back here with some detail and we'll go from there.

Cheers.