I intend to pay it, but I don't intend to devote any time to wondering if mail theft has tripled the penalty because I never received the official document. For Firenze, the fines are a huge income stream, and they are efficiently collected. I would know to watch carefully and pay promptly if that's where I had been fined. Is Prato small? Well, it's famous for biscotti, which most of us have heard of, and some of us have eaten.
Elsewhere in these forums, though, and on other web pages, one can read that many small municipalities do not even send bills to North America. If I were planning such a crime spree as not paying fines, I would choose the dark web for my posts, no?
The document from the rental car company is no more than an explanation of why they charged me €30.50 for giving my information to the Competent Agency in Prato.