The airport taxes changed in early 2007 and are now included in the price of the ticket no mater where you buy the ticket. All airports except those operated by Bangkok Air charge 100 baht for domestic flights and 700 baht for international. Bangkok Air airports at Sukhothai*and Trat charge 200 for domestic and at Koh Samui it is 300 domestic and 500 international.
At Samui there used to be (started in 2004) a service provided by Bangkok air & participating international airlines (except Qantas ) whereby you had the option of not only checking your bags through to your next international destination but clearing customs here rather than in Bangkok. This also saved you from paying the domestic tax of 300 and only having to pay the international one. There was a sign next to the check in desk listing the participating airlines. When your plane landed in Bangkok you were taken to a separate bus and ferried to the international terminal for check in to your next destination. The only requirement is that you had to stay in the transit area as you have already cleared & been stamped out of Thailand. I am not sure that this is still in effect and what the procedures are at the new airport.