If you are only ever going to use MXN, then in your Zen Cart admin, in your Paypal modue settings, you should set the currency to "MXN", or "Selected Currency".

And, in your PayPal account, you need to make sure you have a MXN currency open, and go into your Payment Receiving Preferences, and tell it to always receive transactions in the currency they're submitted as, if you have that currency "open". (I can't remember the exact wording, but it sounds like that.) You don't want it to auto-convert between currencies.