La MesILLA is the Guatemalan side of the Cd Cuauhtemoc CHIS border crossing.
The windiest stretch of highway Mex 200 is the 22 km part between Niltepec and Santo Domingo Zanatepec -- and hoo boy can it blow ! But at least it will be dry , the clouds off the mountain evaporate before your eyes.If it is windy be prepared to lean at 45 degrees while riding straight down the road.
At Tapachula you can get the aduana paperwork done at the terminal at the village of Viva Mexico which is the intersection of the Libramiento to Puerto Madero and that border crossing.If that is all you need you do not have to go farther to the actual border crossing.The other crossing is NE of Tapachula about 15 km and is a crowded mess in the vally bottom.
If you have to spend a night you can just go into Tapachula, fine hotels ,very nice atmoshere around the plaza not a border town at all although there lots of migrants from CA, .You can overnight even without the Mexican papers , it is just like the US side frontier zone idea.