The best thing to do is contact the car hire company ahead of time and let them know you'll be late. Sometimes this is not possible or you are returning the car after operating hours. In this case, you'll be charged a fee for every 30min after the decided initial returning time. Keep in mind that you will also pay tax on this fee, as is the policy with all airports in Italy. After 24 hours, the daily rate of the car is doubled and that becomes your late charge for one or more days. Search car hire on KAYAK