We have decided to stay in Chiang Mai after a year trial. We are going to be shipping our stuff to Thailand. We didn't ship before 6 months because we were renting our home out in USA as a furnished rental. We have now sold the house and are ready to ship some furniture and personal effects. I have been told there is a way I can extend the import exemption on a one time shipment. But I need to know where in Bangkok I go to get that extension document? Or am I SOL? Need to know asap because we are headed to USA in 10 days. It will make a huge difference in what I sell back in USA and what I decide to ship. If I am SOL any idea how customs duty is determined? Is there a place where I can "discuss" the valuation.

Customs being probably the most corrupt department in a corrupt state, it will depend on your negotiating skills. There are official and unofficial ways. I shipped 300Kg of gear by air freight to Chiang Mai a couple of years ago and found the unofficial method to be much more efficient and cheaper.

Assuming you are thinking about seas freight, talk to a few shipping agents in Bangkok and get a feel for the way things are at the moment .

not easy with 10 days to go. Better to have thought about it a bit earlier. Sorry for stating the obvious.

The six months you are talking about only applies to Thais returning from a paid assignment overseas, not to expats who have decided to stay here on a retirement visa

If you are using your Thai wife to qualify for the exemption then I am afraid that you are SOL ,since that exemption is not extendable since the goods must arrive in Thailand no later than six months after you do