Why is there so much spam on this site???
Anyway. As a real answer to the OP.
I guess you're flying into and out of Bangkok and are staying in Bangkok for a few days at either end of your visit to Khao Lak?
If you are simply arriving into Bangkok then going onto Khao Lak why not just fly down there with it?
If it's more of a case of what to do with it when you come back from Khao Lak you can just put it in your room and I don't imagine the hotel would worry about it, as long as you ask them first. The hotels generally have luggage storage areas and they would likely keep it ther free of charge, seeing as you're a resident customer.