This is something I need to do in my country, I have developed resentment towards Sepomex (Servicio Postal Mexicano / Mexican Postal Service), due to the way they treat my packages.
What I hate the most is that companies like DHL that are established in Mexico, will use their own service in the country the delivery is being paid, but when the package reaches Mexico, they give the package to Sepomex, which basically defeats the purpose of using a different company...
In the US some vendors will send using UPS to USPS. I don't know what they save, but I think that the system loses a day or two compared to just UPS or just USPS. This is annoying if it is something that you need soon.
What I hate the most is that companies like DHL that are established in Mexico, will use their own service in the country the delivery is being paid, but when the package reaches Mexico, they give the package to Sepomex, which basically defeats the purpose of using a different company...