I brought up Syntagma (Subject) in another post, but it's sort of gotten buried. I was unable to find out much via Google. Here's the idea.
The Square, according to RS's Athens guide book (+ older video) and, a note somewhere on this site, has a large number of discount places nearby, including air travel. He fleetingly mentioned that one could call some of them directly from the USA. However, gave no further info. I've tried to find out, but zippo. Maybe someone here know the way.