Called Poste Italiane, Italy's postal system is reasonably reliable, though parcels do occasionally go missing.
Francobolli (stamps) are available at post offices and authorised tobacconists (look for the big white-on-black 'T' sign). Since letters often need to be weighed, what you get at the tobacconist for international airmail will occasionally be an approximation of the proper rate. Tobacconists keep regular shop hours.
The cost of sending a letter by aerea (airmail) depends on its weight, size and where it is being sent. Most people use posta prioritaria (priority mail), Italy's most efficient mail service, guaranteed to deliver letters sent to Europe within three working days and to the rest of the world within four to nine working days. Using posta prioritaria, mail up to 50g costs €3.50 within Europe, €4.50 to Africa, Asia and the Americas, and €5.50 to Australia and New Zealand. Mail weighing 51g to 100g costs €4.30 within Europe, €5.20 to Africa, Asia and the Americas, and €7.10 to Australia and New Zealand.