The first letter said, "you are advised to complete...." OP did not. His fine has been increased because he did not complete the form.

As someone who wishes to return to Italy and presumably rent from Hertz in Italy and elsewhere, I would be prepared to pay, but first follow Forrest Bump's advice of a short apology and ask for forgiveness.

However, don't drop it. If you don't hear back, presume that you do owe the money until advised to the contrary.