I have entered everything correctly multiple times, exactly like it's supposed to be, yet it still says it isn't valid. I know the card is valid, I've used it today!
Their store uses AVS, and not all cards support it. It is also a source of issues. Try and put your CC into the web store and see if it helps.
Also, make sure you use the zip+4, there have been a few folks that it fixes their issues.
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