Just get on and pay the driver. They're never very full. [You can't book in advance.]
I THINK you get a small discount if you book on line. Because it's end of season the number of buses running will be reduced, I'd advise checking beforehand. Haves service is excellent, clean, punctual and full insured.
Thank you, Robert. Did just as you said. The bus was almost empty.
Asked at the Havas office at the otogar and they said that I can't buy ticket in advance but from a driver only.
And I didn't find any opportunity to book online.
You're welcome Konclave. It's true that all the major bus companies such as Metro, Pamukkale, Kamil Koc etc offer a discount of about 5 TL if you book online (and they are always full on departure), but that doesn't apply to HAVAS buses which only serve airports.