Do they tell you wish jurisdiction issued the ticket? Most local authorities (DC, Montgomery County, etc) have ticket look up features on their websites. With the licence plate number you can see any outstanding tickets, but they disappear once paid.
For reference in DC going 11-15 mph over the limit will get a $100 ticket, 16-20 mph over gets $150 and its not hard to imagine them charging a $30 admin fee to pay the ticket on your behalf.
I don't think you will have any luck contesting as the rental agreement surely holds you responsible for any tickets incurred during your rental.
FYI- waze and maybe other traffic/map apps have options to alert to things like speed camera, Ive found it helpful in unfamiliar places.