I can't leave the Canadian App Store... HELP!

A while back I got an iTunes Gift Card and put it into my account, not knowing that it was a Canadian gift card. I changed the zip code and was able to use it, but now I can't get back to the U.S. app store. When I try to change the store, it says "You have a store credit balance; you must spend your balance before you can change stores." After some research, I saw I would need to put a credit/debit card into the account, use it to buy something (eliminating the remaining balance on the Canadian gift card), and then I would be able to change the store. However, when I input new cards (one debit, one credit), I received a message saying "Your payment method is not valid in this Store. Please enter a new payment method." How can I leave the Canadian Store? Must I buy/use a Canadian specific credit/debit card. Thanks for the help!