Seems to be it’s impossibile to find out how much the Italian Postal service is charging for a simple shipment of a box outside Italy. In fact, on their site they ONLY show you the more expensive ways (PaccoCelere Internazionale, QuickPak Europe, EMS) but these services have a starting price of € 28 ! There’s NO direct link on their site towards this page: http://www.poste.it/postali/ordinario/estero.shtml
So, if you’re in Italy and you need to ship something, ask for “pacco ordinario per l’estero’. They won’t have any documentation about it and the price will depend on weight, country and if you’ll be using air or ground services.
But how to know what it’ll cost… Well, see the attached image of an internal document of the Poste Italiane! Should be valid from 2011 but your mileage may vary: prices are always subject to change (mostly without notice).