you can just take it to the Post and send it that way ,
I sent a 4 KG box on Tuesday for 1100 baht ($35) but this was surface mail that can take 2-3 months ,
The Thai Post Office has a website in English for prices ,
Also UPS and Fed EX have 10 KG + 25 KG special price boxes , not sure what the prices are on those now but the 25kg one I have used and it took less than a week to get to the USA and was cheaper than the post if you have heavy things and need them in a week or 2 ,
Or you can just pay for an extra piece of check-in luggage which may even be cheaper if you use the full 20kg limit
Soarer