IM disappointed

I rented a car from thrifty , i paid 3500 holding deposit plus rental of 15550 for 2 days. we were driving on the high was on the free way and a small stone hit us on the windscreen , in total thrifty is taking the 3500 holding deposit because of a small crack on the windscreen and an additional 1200 which they say is a shortfall , in total for a small crack on the windscreen I'm paying 4700