Ask them to show you a photo of the type of bus that will take you because I fell in that and I was in a much worse way than the ones I saw arriving at Ranong station with the same origin as mine. Normally the price is the same and the quality, however, it does vary.
I arrived at Ranong bus station at 7.30 am and took a motorcycle taxi -50 Bahts- to the pier from where the ferry leaves.
He ship Slow takes about 2 and a half hours to arrive and costs 150 Bahts, while the speedboat leaves for 350 Bahts and leaves you on the island in an hour and a half.
When I got off the island's pier a small flotilla of motorotaxis They offered to take me to my accommodation. As I had not booked anything I told them to take me to the beach to devote myself to seeing some with the backpack in tow.
If you are in Chumpon - launch port for Koh Tao - there are quite frequent minivans that make the trip to Ranong in a couple of hours for about 140 or 160 Bahts.