Well I finally found the page with the details. It's at the very bottom of here:
https://rentacar.aegeanair.com/en/benefits/
Unfortunately neither of the phone numbers work
For reservations please call 801 11 20000 ( from Greece) or + 30 210 6261947 ( from mobile or out of Greece).
. When I therefore call Aegean directly their telephone agent tells me to call Hertz...providing me with a phone number....Hertz then tell me to call Aegean...providing me with another number.
It's turning out to be a great benefit
