Do you plan on visiting Mexico for more then 7 days? Prepare to pay a new tax. Currently this only is for foot traffic, but how long before it applies to vehicles? Ideas like these only grow with time...
If you are flying out you need an FMM and it may cost another $20-something. Driving out you don't need anything
The fee this post is about is the FMM (tourist permit) and businesses thought ID checks and possibly needing an FMM would slow business. It's always been required going further into Mexico