If you need to send a mail urgently, the Italian Post (http://www.posteitaliane.it/) provides a fast mail service (Posta Celere) in most of the post offices, an warranty to deliver within 24 hours in Italian towns and within 48 hours in the rest of the country; to send abroad, it takes 24 to 72 hours in European countries and 3 to 5 days in the rest of the world.
You can buy stamps in tobacco shops or in post offices. They are open from 8.30am to 2pm from Monday to Friday and from 8.30am to 12pm on Saturday. Main post offices are also open during afternoon.