Reservation fees apply for trains that require reservations, in Italy this is €10 for Frecce… trains and €3 for Intercity IC trains.
Also, as this seems to be a regular misunderstanding, One country passes are only for travel within that country. To get to/from Italy you will need either to buy a Global pass or pay for those journeys separately.
If you are only making a single trip to Italy and back may be cheaper to buy this as regular tickets.
If your travel while in Italy will involve multiple days of trips on longer train journeys then Interrail may save you money over regular tickets. If you will only use local or regional trains then buying regular tickets which are cheap for local travel or using the Trenitalia - “Italy on tour” ticket https://www.trenitalia.com/it/treni_regionali/promo-italia-in-tour.html may be better value.