Mae Sai/Tachilek border is closed to non-Thais and non-Burmese.
If you use Chiang Saen border to cross to Laos by boat (Golden Triangle Special Economic Zone), you have to stay at least 1 night. 2000 baht entry for non-ASEAN citizens.
I would just do research of that area of Laos and you'll see why.
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