My agent informed me anytime you return back to Thailand you need to do a new TM 30 in person or with the aid of an agent at an immigration office in Thailand. It does not matter if you are away for a short period of time and returning to the same condo
Regular postage from Thailand to USA is under $5 for the envelope you described. Go to a Thai postal Outlet not a outlet that deals in different couriers. I did it took less than a week to get to Canada