What documents and fees do I need for a 90-day border run at Mae Sai for my NON-O marriage visa?

Cleaned up, there are no reports of Mea Sai border is asking for proofs of cash etc. He has the 1 year Multiple Entry Non Immigrant O visa based on married with a Thai National and is going for a new 90 day period of stay. Not a new visa exempt entry or new tourist visa. Please read before you all start about different entry points in Thailand and people having long histories of using visa exempt and tourist visa.

Mae Sai border immigration won't let you out if they won't let you in again because the Myanmar side don't give you a visa, they give you a day pass so you would be stranded in Tachileik. Also, you're not doing any kind of '90 day check' as in 90 day reporting (which reports the fact that you've been in the country for 90 days or longer) because you haven't been in the country for longer than 90 days, and can't on a visa that requires you to leave the country every 90 days. You're leaving Thailand and entering again which will get you stamped in for another 90 days, Mae Sai also don't ask for money, onward travel, or accommodation bookings.

Myanmar immigration want ฿500. They keep your passport and give you the day pass. Tell them "No shopping" and they will be quicker than usual in stamping you in, and out, and sending your passport over to the office on the other side of the road where you pick it up on your way back to Thailand. When you reach the Thai border again pick up anew TM6 at the little kiosk in the middle (fill it out on the little bench which has pens on it) before you get in the queue for the entry stamp.