Additional costs
We read and heard a lot about that. In November 2017 (beginning of the month), we had to pay 500 MEX$ per person to exit Mexico. According to the van driver, this amount is due to people who stayed more than 7 days in Mexico. Nothing appears officially and to be honest with you, the fee did not look legal at all for us but we did not have choice.
In Guatemala, it was the same. We had to pay 2$ per person to enter the territory. No idea if it was legal because no document, no receipt, nothing… but well, hard to not pay.
So here it was our practical information of the day. Very easy but a bit confusing.