I understand that you recently redeemed an iTunes gift card that applied credit to your account, but when making a purchase you saw a charge on your credit card. First, you shouldn't have to do anything for iTunes to use your credit first:
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.
If that did not happen in your case, I would check two things. One, make sure you redeemed the card by following these steps:
If you're sure the credit is applied to your account, then I would check and see if the charge you're seeing on your card is just a payment card authorization hold and not an actual charge:
Welcome to Apple Support Communities!
Have a great day,