Can I combine Apple Gift Cards and credit cards to purchase apps from the iTunes Store?

First: Redeem your gift card onto your Apple Account balance.

Then: When you make your purchase, your account balance will be used first; with the remainder charged to your “next in line” account payment method (i.e. your credit card).

How to redeem your Apple Gift Card, App Store Card, or App Store & iTunes Gift Card - Apple Support

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Most in-app purchases can be made using the Apple Account balance from your gift cards, but there are exceptions. See info in here: What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

What are you attempting to purchase? Review the information provided here for what it cannot be used for:

• You can't use your Apple Account balance to send gifts or gift cards.
• For some recurring charges, like AppleCare+, you can't start your subscription with Apple Account balance. But as long as you keep another valid payment method on file and your Apple Account balance would cover an entire charge, your recurring charge will be pulled from your Apple Account balance.*
• Some subscriptions might not be charged to your Apple Account balance.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are available only to you. Learn more about how purchases bill to your Apple Account balance.