I don't know what you mean by your "iTunes card". The only card named an iTunes card is a gift card and when you redeem that to an account all the balance is transferred to that account and is added to your Apple ID balance. The card is essentially useless at that point because there's nothing on it. If you mean the gift card, Apple doesn't take the money from that, it can only take it from your Apple ID balance once the card "becomes" your Apple ID balance.
So I guess I am becoming confused by terminology. ;-)
Here's something that may help you understand how this all works:
How App Store and iTunes Store purchases are billed. Click here --> https://support.apple.com/HT201359
"Which payment method is charged?
When you buy items or pay for subscriptions, Apple automatically 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, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, 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.
Some purchases aren’t charged to your Apple ID balance
- If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method.
- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.
- Some subscriptions might not be charged to your Apple ID balance.
If none of your payment methods can be charged
If a purchase can't be charged to any of your payment methods, your account has a negative balance. You can't buy more items or update your apps until you update your payment method."
