The Havas service (and timetable) is fairly simple. Two buses meet incoming THY domestic flights and go Fethiye and Marmaris.
From Fethiye, they leave two and a half hours before departure time (of a THY domestic flight) from the Otogar (they used to go to/from Karagozler but dont any more).
If, as in the past, Onur, Atlas, Pegasus etc do summer services to Dalaman then they usually meet those flights too.
They only leave from/drop off at the domestic terminal and to Fethiye, and 20TL per person sounds about right.
JF