Le Poste (www.poste.it), Italy's postal system, is reasonably reliable, but if you are sending a package you might want to use DHL or FedEx, which can be safer.
Francobolli (stamps) are available at post offices and authorised tabacchi (tobacconists; look for the official sign: a big 'T', often white on black). 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. If you've any concerns about ensuring an accurate stamp price, use a post office. Tabacchi keep regular shop hours.