Actually it is quite simple. Of course you must pay for shipping so it depends on how much you wish to bring. At customs there are different tariffs for each category of items. Strangely tools and musical instruments have high duties. Everything must be used. In practice the importer will negotiate a composite rate based on the manifest. I have imported two separate containers pf personal effects and in each case the duty came to about 10,000 THB. I much prefer paying this amount to spending days slogging around shops finding replacements are expensive or not available - especially, books, art works, art materials. If you only have a few t-shirts and jeans with holes then yes, don't bother.