Im in Mexico and customs isnt too bad. They dont seem to care what you are briging, but if your package is stopped they are more concerned with screwing you for some money to free the package.
Mine was sent through DHL and actually went through Cincinnati, I almost crapped when it hit the US first. Usualy from China it comes straight to Mexico City then sorted.
DHL and FedEx are poor choices since they seem to check everything. EMS enters Mexico through the local mail system and seems to just be random.
In my case DHL is requesting an import license even after supplying them with all technical documentation. Its a joke.