Account credit from iTunes card

If you're gifting items or contributing to a disaster relief campaign, iTunes Store credit isn't valid for those transactions.

If they're purchases by a Family Sharing subaccount, the organizer's store credit can't be used.

If neither of the above apply, click here and ask the iTunes Store staff for assistance.

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According to Change, add, or remove Apple ID payment methods - Apple Support:

"Apple will attempt to charge payment methods in order from top to bottom. You can't change the order in which Apple ID balance is charged."