Just been back from Samui and perhaps share the experience. There is no fuss going from Surat Thani to Donsak and taking a most enjoyable ferry across. First, there are enough counters at the airport that will arrange for you a smooth ride over to samui. The journey is approximately 1.5 hours ferry and 1.5 hours bus ride. Everything can be arranged at the airport and needs no prior overpriced online booking. Return from Samui to catch a flight is easy too. I caught my 1pm flight and have ample time to grab a bite at the airport before taking off.
At Samui, plenty of travel companies to arrange for you to get back to Surat Thani to catch you flight. Cheaper than if you organize a return ticket from Surat Thani (50-70 baht less). Phantip Travel charges 550 Baht per trip/1100 baht return.
However, please do not throw away any ticket stubs, loose receipts etc - the drivers will take advantage of the opportunity to extort some money even if you lose a useless ticket stub. The journey from Samui ferry terminal to Chaweng/Lamai depends on the jams (there are!). But generally Samui residents are nice people, FAR nicer than some tourists.