Without much definitive information on this subject, I paid extra attention on my crossing to get as much info as possible:
Air Aran Pattana Bus Company
Reservations: 089-8449624
www.ok-bus.com
Departures: 10:00 / 13:00 / 16:30
Duration: Approx 5.5hrs
Price: 200b
To get to the bus location, after crossing into Thailand, continue walking the same direction through customs and make a right at the 1st main street (not into the market). Continue on this road approx. 5km until the pavement ends. You will see a large white billboard on the right as you walk. You can use this as a distance reference as the depot is located in the shopping complex. Easy to walk and 40-60b via tuk tuk.
Hope this helps.