mexico isn't a big deal at all!
We fly lears in and out of mexico all the time with freight and air ambulance, pax a little less, but nonetheless. We started out having a handler but that ends up just being a big cost that ou don't really need, nowadays we do everything ourself, just make sure that at anycustoms office you go to, take a few papers with you to have in your airplane, make copies if you just want a bunch of empty forms. We fill them out on the flight over there, and we plan the flight just like any other flight, look up the miles in the GPS, look at the high-chart and find some airways until you hit the US border,from there just /g direct.
I have hardly ever ran into problems in Mexico, and ABSOLUTELY never went any dinero out of pocket for services that I needed. And I won't either. I need to sa that I hardly speak spanish, but you'll catch on to what they are saying in front of you soon enough.
All in all, it seems that everywhere outside the USA The (local)aviation authority and customs seem to work very close together. As opposed to the US where the FAA and the customsservice have absolutely nothing in common with eachother.