I want to go from Athens to Chania (Crete island) by the night ferry on Sep.12, 2015.
I cannot pay online!
The payment is never successful.
So, possibly i should give up and just buy the ticket one day or two days before from the agency?
September is starting to be low season especially for islands in Greece. No need to book them.
Also all these years I have found web sites to be unreliable when booking online, not so much for the fact that you can or not but sometimes the timetables are plainly just wrong! What I would do is call them. They will probably speak English. If not get to Greece and get somebody to do it for you. For me it's a holy rule.
Greek boat timetables = phone call!
Most probably you won't have any issue purchasing a ticket last minute from an agency or the port. You can also call them and refer to your issue. For sure there are alternative ways to pay.
You can also reserve a ticket when you call them and pay it later, at the agency when you arrive in Greece. The unsuccessful payments might be due to capital control measures but it's uncertain. Try other sites.
Keep in mind there are some pretty cheap flights from Athens to Chania.