Q: Always invalid

Was the card issued in the country where your account is based (gift cards can't be used with accounts based in other countries) ? If it was issued in your country and you are getting a message that the code is invalid (how to redeem a card) then try contacting iTunes support and see if they can help : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page