The site is very poorly maintained, it seems. I did try to purchase tickets (just for the Lernardo Express, so no big deal), and it just kept saying that my payment request was not accepted. No reason, no corrective action, nothing. I tried 3 different credit cards held by two different people. Maybe it's just that it just doesn't like US cards, as you say, but it certainly gave no indication of a reason. Unless they get around to fixing this, buying in advance may not be an option.