Hi rfrassoni,
I don't have any experience from Mexico to US, but I have shipped from US to Mexico for business often.
If it's a letter, document, or book , definitely consider using FedEx, DHL, UPS or similar service. They ship quickly and relatively problem-free.
Again, I don't know about the Mexican postal service but US First Class International Service is much, much cheaper for packages. It can take much
longer though. I've had things take more than a month to get to Mexican towns a few hundred miles away from me. My advise is to just let the customer know that it is: pay much more or it can take up to a month.
Hope this helps you. Good Luck.
