thunderbird27, welcome to FT and the A3 board!
I would personally doubt the check-in or despatch staff will be particularly interested in whether or not you have the car seat - it’ll almost certainly fall to the cabin crew to enforce this because it’s a flight safety/passenger safety issue which is more in their domain than anyone else’s.
Different airlines have slightly different requirements on car seats, particularly across continents. I would check that what you have conforms to A3’s requirements - in spite of the mention of “international” this isn’t necessarily the reality!
The link is:
https://en.aegeanair.com/travel-info...paring-to-fly/