I agree: call the agency.
Worth noting: Having an IDP (or equivalent translation) is not a legal requirement to RENT a car. It is a legal requirement to DRIVE a car (for a foreigner). I've rented cars in Italy many times, from many different agencies, and have never been asked by the agency to show an IDP (I had one, of course, just never had to show it to the car rental agency).
So it's very possible (likely, in fact) that the agency won't care - IME they usually don't - because they don't have to care. If you are stopped by police or worse, if you get in an accident, and you do not meet the legal requirements to drive in Italy, it's not the agency's problem, it's entirely yours (they would get their car back, you would have legal troubles). So they really have no skin in the game and IME typically do not care if you have an IDP/translation or not. Of course, you might find the odd agency or employee who does care, but that seems to be luck of the draw, and not something to count on.
So yes, it's probably worth finding out how picky your agency is going to be, so there's no surprise when you pick up the car. But it's actually the police/legal system that mandates having the IDP or equivalent (and hopefully, you won't have to find out how picky they might be).
Good luck and drive carefully.