I tried ordering a card last night for a friend - ran up against the same brick wall, wouldn't take any of my US cards.
I wrote their customer service an email, and they immediately offered to send me a card for free (presumably with no credit on the card.)
I would like this company to succeed, because they have a great service. While they are not there on data yet, their prices for voice in many cases are (much) cheaper than a local SIM card. So let's all be nice Flyertalkers and only write to their customer service if we've tried ordering online first and the order was rejected.