I found 3 facilities to recharge italian SIMs from abroad (even with PayPal).
The websites are in german, the companies are situated in Südtirol.
These shops also sell italian Prepaid SIMs.
www.messaxio.com (costs 5€ fee) ("Ricarica/Aufladung")
SaveonRoaming.net - Ihr Spezialist für mobiles Internet - Prepaid SIM-Karten Italien - Guthaben für alle italienische Provider aufladen costs 3,90€ fee ("Guthaben aufladen")
www.elektrohaller.it (only TIM, "TIM Wertkarten")