The best thing to do is pay it as soon as possible, preferably while you’re still in Italy. If you’ve returned home, you can pay online. If you receive a fine in the mail, check to see if the car rental company has already paid it (with your credit card). While not common, this does happen. The ticket will have instructions, and you can pay online with a credit card. If you believe you have received the ticket in error, you can follow the instructions on the ticket to dispute it.