iTunes card before credit card
How do I use a iTunes card to purchase something before my credit card?
iPhone 12 Pro Max
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The iTunes, App and Book Stores will automatically use any redeemed iTunes Gift card credit balance on your account before charging your credit card. There is nothing specifically you need to do beyond redeeming the iTunes gift card to your account: link-> How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support
It will however still require the existing credit card be valid and verified before using the iTunes gift card balance.
It will not charge the credit card once verified and will use the gift card balance directly as long as there is enough balance to cover the purchase and any sales taxes. If there is not enough to cover the total cost, any remainder will be charged to the default payment method on file.