If goods are imported via post, e.g. via EMS, then if the total value incl shipping is less than 40,000 baht, and no inspection is required i.e. customs know what the goods are and what tariff is applied, then the Post Office can deliver to you but you still need to pay the import duty and VAT. Otherwise i.e. goods are too high value or need to be inspected, you have to go to the nearest customs house to have the goods inspected and pay the import duties and VAT. For goods purchased from ASEAN countries there is a trade agreement - see https://www.thailandntr.com/en/goods/tariff
I've had to go to the customs house 3 times to collect items sent via EMS and each time the customs officers were friendly and polite and I was always charged the correct duty according to the tariff (my stuff was bought from HK).
I've had just one experience with DHL which was a few months ago. They had staff shortages due to COVID and it took much longer than it should but they cleared customs on the day of arrival, the import tariff and VAT were the correct rates and their own charges were clearly listed. Yes, they were much more than the 20 baht fee I would pay at the post office but they delivered to my door and I didn't have to drive 2 hours to the customs house and spend half a day there.