I can't say if everyone over-charges for transactions in foreign currency as much as Aegean does (which is a lot!), but pretty much everyone over-charges more or less in such cases (one easy example is Amazon, even if it's by something like 2% more or so).

Basically all you needed to do was to try and switch currency on Aegean's website (to Euro), then use a currency exchange rates source (xe comes to mind) and the difference would have been immediately clear. Since this is common practice among businesses of all kinds it's very unfortunate you didn't realize it before making the booking.

I don't think getting a receipt in usd would have helped much either, I think the 24h rule is a strictly US thing (unfortunately for everyone else too) and I don't believe that simply visiting Aegean's website and paying in dollars qualifies as a tickets sale within the actual country.

I also doubt there would have been any clear indication of a "currency exchange fee" or something like that in the receipt, they would have simply written down the prices in usd ss they would have in eur, only they would be something like 10% than what the actual exchange rate would dictate (plus braking down the total amount into smaller ones in the detailed receipt would actually make it more difficult to spot the difference in the exchange rate).