Q: How pay apps with iTunes gift card?

Hi Chiquita Capuccina,

Welcome to the Support Communities!

This article will explain how payments are processed:

iTunes Store: How iTunes Store purchases are billed

http://support.apple.com/kb/HT5582

Order of iTunes billing methods

When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.

This article will show you how to review your iTunes Store account and update your billing information. 

iTunes Store: Changing your payment information

http://support.apple.com/kb/ht1918

Cheers,

- Judy