Why is my Apple Gift Card code/PIN not working?

APPLE Advisor on the phone said to use the PIN # on the card " and ususally starts with GCA",

and click "check my balance".

Several attempts and does not work. Very concerned as each card was purchased for at total of $500 each - So if just lost $1000.00 dollars between two Apple Gift Cards and triple checked GCA PIN Number - it becomes beyond aggrevating...How can I check the unopened - unused APPLE GIFT CARDS and check the balance of each, before I blow a gasket (my Brain)

[Edited by Moderator]