Originally Posted by Jamie Z
To return this thread to Road Wisdom...
I just returned to the US from Mexico after transiting most of Central America. I crossed at Laredo, bridge #2. I hadn't yet paid my $20 tourist visa, and nobody ever asked for it when I crossed the border. $20 saved.
Since the visas are multiple entry, they don't ask for it when you leave, you have to go to the immigration section (not always the same place as Banejercito/vehicle import permit, not sure what it's like in Laredo) and return it to get an exit stamp in your passport.
You may get asked about it next time you go to Mexico, depending on how diligent the officer you get is. If you didn't return it/get an exit stamp, they'll consider you as if you had overstayed your visa, most likely you'll have to pay a fine. I doubt you'll end up saving any money.