Using iTunes Gift Card for in-app purchases

How items are paid for is automatic. Have you gone through with the IAP purchase ? Though you may be being asked to confirm, or correct, your card's details it doesn't necessarily mean that the card will be charged (except for a temporary store holding charge) - confirming the card's details is a way of checking that it's your account. If it does charge the card contact iTunes Support

That article didn't help, I have same problem/question. There is a free app: trip view (train timetables) lite but I want to upgrade to full version. It costs 4.95 or something and I have $50 of iTunes credit in my account. But every time I try to purchase the full version it takes me to credit card payment. I want to pay the 4.95 in my current iTunes balance instead. HELP

If you use a gift card for a transaction, the amount is deducted at the time of your transaction.

When you make a purchase, content credits are used first, as long as the value of the item that you're buying is less than or equal to the value of the credits. Any gift cards are used next. If your gift card is less than the total cost of the purchase, the payment method that you have on file is billed for the remaining balance.

If you have no content credits or gift cards, the payment method that you have on file is charged for the entire purchase.

When you gift an item to someone else, no credits are used. The payment method that you have on file is billed for the entire purchase.

Hi

The following support article explains how iTunes Store (including App Store) purchases are billed:

How iTunes Store purchases are billed - Apple Support