How to change payment so my gift card won’t be used up

For more specifics on what Niel outlined, read this support article: How apps, content, and subscriptions from Apple are billed - Apple Support

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.

Except by gifting the content to yourself, you can't. Available iTunes Store credit will be used before any other payment method when allowed.

(190183)