Italians don’t tip. It’s not customary to do so. There is no space in the credit card slip to add a tip amount, as common in the US. The reason is also tax and fee related. If it goes in the credit card, the owner has to pay VAT to the government for that amount (10% for hospitality establishments) in addition to the 2% typical card fee.
The only place in Italy where I ever saw the gratuity line in the card slip is the restaurant of the Hilton Airport hotel in Rome FCO airport, a hotel patronized by 90% American flyers sleeping there the night before the flight home (I do also so sometime). My (American) wife wanted to leave a small tip at that Hilton restaurant against the advice of her Italian husband (me). But when she was about to add the amount to the card slip, the waiter stopped her and asked her that if she wanted to leave some tip to please leave it in cash because if placed in the card, Hilton would keep the money and give zero to the servers. When he told us that, I realized that adding the tip line was just a gimmick by Hilton to take advantage of their American clientele to make extra profit. I have never seen the tip line anywhere else in Italy, but if someone started that practice, I’m sure that gratuity would go to the owner and zero to the servers. As I said Italians don’t leave tips, except for rare occasions for exceptional service and usually only in upscale restaurants, but it’s always cash money given directly to the server or left on the table. It would never be more than 5-10% (basically a few euros). Also consider that Italians do not ask for the check at the table. They simply get up and on their way out of the restaurant they pay at the cash register (usually manned by the owner). Do not leave the tip at the cash register (with the owner) or the owner will keep the whole of it and the server will see none of it. Leave it on the table or better yet give it directly to your server on your way out. It’s not customary for Italians do leave tips but servers are not going to be offended or refuse it if you do. They know Americans are generous (while Italians are cheap bast…., as some friends in Italy who are servers tell me).