score:5
It's probably just that they had an hick-up in their system, and were not able to process your request. That can happen, I had it just last week with Air France, buy one ticket, fine, buy second ticket, failed, with any of four cards. Next day it worked fine.
Alternatively, (all) your cards do not allow international transactions. Your issuing bank would be able to tell you about that; and potentially change it; but it looks like this is not your issue.