@Tom *****
You will have NO problem at all border bouncing at the Mae Sai - Tachiliek border with Myanmar
It cost 500 baht to get stamped in and out of Myanmar, and you'll get a 30 day visa exempt entry stamp when you stamp back into Thailand.
You can also extend that 30 day visa exempt entry by another 30 days at the immigration office for 1900baht.
Good Luck (y)