Letters sent within the country cost S/3 for less than 20 grams; letters and cards up to 20 grams sent to the United States and Canada cost S/8.20. Bring packages to the post office unsealed, as you must show the contents to postal workers. Mail service has been improving, and a letter should reach just about anywhere in a week from any of the main cities. For timely delivery or valuable parcels, use FedEx, DHL, or UPS, although be aware that customs can be very complicated.
DHL, FedEx, and UPS all have offices in Peru. Because of the limited number of international flights, overnight service is usually not available.