You can use the $50 credit, no matter where you are (it doesn't ask for a Credit Card until after you run through the $50).
After that, there are ways. If you know someone who has a U.S. addressed card who will let you use it, then that'll work. And several extra-U.S. MobileReaders have confirmed that other gift certificates work just fine, if you can find someone to purchase and send them to you. There's been some traffic around here in that too.
