The billing failed when you put the US address? That's strange. I put in the shipping address for my billing address and it accepted it, no problem. (Though I didn't go through USUnlocked.)
Are you sure they said the billing failed? Was it cancelled? Or did they say "Unable to process order, please call 1-800-WE-SUCK" or something similar? If it's the latter, don't worry about it. They sent me the same thing. Just contact them* to confirm/verify your order, and they'll ship it.
* Yes, you'll have to call, unfortunately. I tried writing them an email, but it looks like you'll have to actually speak to the Audit people on the number provided. On the bright side, speaking to them on the phone is certainly quicker than writing them an email and hoping they check it soon.