You might have gotten bad info from Ryan. My son bought 2 different round-trip tickets from them (each for two people) a couple months ago, using a US credit card. Although a new web site or policy change could be causing your problem now?
Romelle
FYI - we also learned you can't do on-line check-in when you have a US passport, and then when you do it at the airport they charge a 3 or 4 Euro fee for each leg. One member here said the fee was supposed to be waived if you only had carry-on luggage, but we didn't know that at the time so just paid up.