Slight update to Janet’s info. The airport buses now don’t have punch validation: the tickets have been changed to electronic and you need to hold them against a small screen.
This is part of a general change to all ticketing in Athens. There are now barriers at almost all metro stations and you will need to swipe tickets on entry. Similar electronic validation is also being installed on buses and trolleys.
BUT, it’s anybody’s guess when all of this will start working. It was scheduled for last January but has been being put back regularly. The airport buses seem to be the only place where the new system is in play.
More details here for anyone interested