Have you tried
Kobobooks?
I've had no difficulty ordering from them (from France, but with a US credit card - billing address on the card is still France, though). Their selection is maybe not as extensive as the Sony Bookstore in the US, though their prices are better on many items and they send out periodic coupons which seem to be applicable to just about any book on their site. I also find they have an interesting mix of US and UK books that generally aren't available through the US sources.