Hello akplrbear,
Thanks for using Apple Support Communities.
For more information on this, take a look at:
iTunes Store: How iTunes Store purchases are billed
http://support.apple.com/kb/HT5582
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.
Best of luck,
Mario