I read on a shipping site that customs exemption was for items less than 1,500 baht and less than a kilo in wieght. Unless they contain prohibited or restricted items. I try to keep packages small and light and never have a problem receiving them.
The exception to the exemption is when your package arrives with UPS or DHL. They like to create problems that don't exsist. In my experience UPS is the worst offender. But I read others here saying DHL is bad. I have had many packages arrive with DHL no problem at all. A couple of things DHL sent me an email and said they needed a copy of an ID, a description in Thai of the item, and a purchase receipt copy. But I received them no problem after that. UPS has on a few occasions demanded that I either mail them my passport or bring it to the customs office at the airport to get my package delivered. Once was for a box of candy canes.