I came in on a tourist stamp 30 day. My first Non O Visa had a 15 months expiry date this because I got a 90 day extension before the 12 month non O Visa. This is how it was done for me when I first got the Non O Visa. Now Non O visa renewal is 12 months each time. When I got the first non o visa I also got a Thai car and Motor bike license, a bank account and a multiple entry permit and they due 90 day check-ins for me all for the 50k I mentioned originally. For me this a good way to go because I don't want to stand in line ups wondering if all my ducks are in a row. The agent does everything for me. I only hand my passport to him for renewal once per year and pick it up when he has completed with immigration for me.
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