Just used the Air Asia all in one ticket from Koh Phangan to Surathani
It was well timed and convenient
Believe they over same service to and from Koh Samui.
You simply choose the island as your destination or start and they then include a transfer from the airport to the pier and a ticket for the ferry.
I had not known about this when arriving and it was a hassle to get from the airport to the pier by myself and then I just missed a ferry and had to wait hours for the next.
When I booked the whole lot via Air Asia, everything was timed to connect and it was a lot easier.
Recommend it.