The privatized postal system is run by Serpost (www.serpost.com.pe). Its service is fairly efficient and reliable, but surprisingly expensive. Most international mail will take about two weeks to arrive from Lima; longer from the provinces.
A post-office box is known as an apartado postal (abbreviated ‘Apartado,’ ‘Apto’ or ‘AP’) or a casilla postal (‘Casilla’ or ‘CP’). Some addresses have s/n (short for sin numero, or ‘without a number’) or cuadra (‘block,’ eg Block 4) after the street name.
Only addresses in Lima and neighboring Callao require postal codes. Those used most often by travelers are Lima 1 (Central Lima), Lima 4 (Barranco), Lima 18 (Miraflores) and Lima 27 (San Isidro). Note that the word ‘Lima’ is essential to these postal codes.