Q: How to use iTunes card and not credit card informat

Read http://support.apple.com/kb/ht1574

iTunes Store: How iTunes Store purchases are billed

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

Check the balance of any redeemed iTunes card just in case > Redeem your Apple Store Gift Cards - Apple Store

If there are not enough funds available, that's why your credit card is being charged.

Take in to account when purchasing any applicable taxes or fees added to the purchase price.

iTUNES STORE - TERMS AND CONDITIONS