Hallo kha,

I've been having a lot of problem with the tax and the Thai custom kha, since I've lived in the US and always ship my merchandise through EMS to Thailand and most of the time, it's got caught at the custom. No matter how I define it on the package na kha or where I ship it to, the residential address or the work office (I declare it as a gift everytime but never get passed their sharp eyes kha).
Anyway back to your question, since the tax amount that they charge me is so unpredictable sometimes less, sometimes more, I then always reasonably estimate the tax and include it into the sale price and since all of my merchandise'll be shipped to my friend first, she'll take care of it (pay the tax and get my bag from the postal office) and and I'll pay her back kha.
Another thing I heard and it may be useful is if you send it with the priority mail or registered mail form the US, your package's rarely checked kha and that may give you more chance to have your item gets through smoothly, but who can wait for that long shipping process though.