It's possible to enter Myanmar at the Mae Sai border post. A 14 day visa is USD10 or 500 baht. Note, if your USD10 is not in pristine condition, the authorities will not accept it. They really want 500 baht instead since it is worth USD16. As of October 2017, $10 is readily accepted again and the notes no longer need to be pristine. The visa is valid only for the Tachilek-Kengtung area and is not for travel to other parts of Myanmar. Note, the Myanmar Immigration authorities will hold on to your passport for the duration of your stay in the country giving you an entry permit instead. You will get your passport back on crossing back over to Thailand. Holders of a normal one-month Myanmar visa cannot use it to enter at Mae Sai. As of November 2017, foreigners holding Myanmar e-visas can enter the country at Tachilek. Ensure you have the appropriate travel documents to re-enter Thailand at Mae Sai, you will only be able to get a 15 day entry pass into Thailand rather than the usual 30 day one that you get at the airport. If you have a visa and want to keep it, remember to get a re-entry permit in advance. As of July 2016 even 15 day entry Thai entry stamps are not being issued and a large sign at the border stresses that the crossing cannot be used by visa-runners. If you are a Thai citizen, you can get a temporary border pass at the Immigration office about 2 km before the border on Pahonyotin Rd in Mae Sai for 30 baht. All you need is your bat prachachon (ID card).