score:3
The Myanmar entry permits granted at Mae Sai/Tachileik are not full-fledged visas: they're only good for 14 days and do not allow travel elsewhere in Myanmar, and this is enforced by them keeping your passport! On the upside, they're granted to anybody with a crisp 10-spot and a pulse, and I suspect that second requirement is optional. Details at Wikivoyage.
And it's a good thing you do have that Thai double-entry visa, since Immigration has recently (May 2014) been clamping down pretty hard on visa runs. Obviously this only applies to people abusing visas on arrival though, and I'm kind of curious about what they can realistically do to a busted visa runner returning from Tachileik, as there's literally no other way out than through Thailand -- probably a toss between a short this-is-your-final-entry stamp, or a spell in Immigration detention before they deport you through Bangkok.
Upvote:2
There are no documentation requirements to cross into Myanmar for the day from Mae Sai. It costs US$10 or 500 baht (better value in $). The signs say baht, but if you ask the $10 option is available.