In 2018 we arrived on Koh Samui with the ferry and we left Koh Samui by air, flying with Bangkok Airways. The airport of Koh Samui (USM) is privately owned by Bangkok Airways and all domestic flights to & from Koh Samui are done by Bangkok Airways. That is why the flights are so expensive. Bangkok Airways is flying from Koh Samui to Bangkok Suvarnabhumi Airport.
Koh Samui Airport is a bit different as other airports as it is called a boutique airport. All buildings are ground-level and open-air. There is 1 building for checking-in and then you will have to walk to the gate. Before entering the area of the gates you will go thru security. The gate is also open-air, what is a bit complicated when it rains a lot. While walking to the gate there are little shops to sell overprices food & drinks. From the gate you go by open-air electric tram to the airplane.
There are many flights going from Koh Samui Airport, most are going to Bangkok. So it was quit busy when we arrived at the check-in terminal. So busy that it was a bit chaotic with long lines for checking-in. If you do not yet have a boarding-card you can print one at one of the machines that are at the front side of the terminal. Then there are 2 check-in rows, one for people with boarding-card and one for people without boarding-card. It took us about 30-45min to drop our check-in luggage. Then we walked to our gate. The flight itself took about 1 hour and was comfortabel and smooth. We were even served a kind of lunch of black spaghetti with duck. For more information see Bangkok Airways. For more information about Koh Samui have a look at Camille's Samui Info Blog.