I have always used EMS - Express Mail Service - with Serpost. Once it is sealed by printed tape at the post office, In my experience has never been open again. But it could be open if someone think something inside is illegal or dangerous
Serpost works as follow: In the area where I am, there is an small office. Every day, the gathered packages are picked up by a van and sent to a bigger office (administracion). you might skip less handling by sending your package to the bigger office.
In this list there are some addresses of administracion and sucursal all together. Notice that suc. = sucursal = small office http://www.serpost.com.pe/oficinaCorreoGiro.html