I can't believe that any parking ticket (or any other type of "Traffic Fine") in Paris would be only 20 Euro.

I can however believe that Hertz would charge you 20 Euro to provide your details as the responsible driver for a ticket if they were asked to. This would also explain why they can't give you details of the "fine", as that's not what you are being charged for.

It's possible you'll never actually get the fine, if the relevant authoritative decide it's not worth following up for a foreign address. Or it might just still be on it's way to you...