The national Peruvian postal service 'Serpost' is much better than its reputation. Receiving mail in Lima and sending it from Lima elsewhere is quite reliable, while outside the capital it's less dependable.
Sending mail within Lima and from Lima to the rest of the world is quite reliable, even if exceptions confirm the rule. Outside the capital, the postal service is unfortunately less consistent. If you want to make sure your mail really reaches the recipient in Lima, Peru or elsewhere in the world, better send it as registered mail ("registrado" or "certificado").
Airmail to Europe or North America takes up to two weeks, packages and parcels can take around four to six weeks; at least they normally reach their destination. Postal charges for sending postcards and letters to international locations are reasonable, parcel are a bit more expensive depending on weight and destination.
Letters and smaller parcels are usually delivered directly to your address in Lima. Bigger parcels and packages have to be picked up at the nearest customs post office after receiving a notification. Advise the sender to spend a few more bucks and send your envelopes or parcels as registered mail. This way we received all mail send to us from overseas in the last years. Mail from North America and Europe send to Lima takes around 10 to 14 days.
Note: Articles for your personal use worth less than US$100.- can be received paying no custom fees. Sometimes the postal charges from abroad are added to the value.
For those without an address in Lima, they can send mail to general delivery. To pick up your mail, you need your passport. As letters are sometimes sorted and filed strangely, make sure you have them check thoroughly for your mail. Normally letters are kept 3 months before being sent back, registered mail sometimes only a week or two. Although this service seems to work well, it isn't 100% reliable. Make sure they address the letter as below:
Serpost has offices spread around town. We can find a listing of all post offices in Lima on their web page under "red de oficinas". The page is only available in Spanish. For "tariffs" and "sending options" have a look under "tarifas".