Complaint Review: Enterprise Rent-A-Car

I rented a car from Enterprise with two of my friends in Colorado Springs, CO. It was -35 degrees outside the day we got the car and we were told to inspect the vehicle before leaving.  We had to walk half way across the parking lot and when we got to the car we all quickly walked around it looking for damage but didn't see anything.

We drove the car to the Enterprise in Denver, CO, which is an hour and a half drive.  Once we stepped out of the car the employee said that there was damage.  We all asked where and he had to point out this tiny little dent in the hood that was still hard to see even after he pointed it out.  I called the store in Colorado Springs and the person I spoke to said it wouldn't be an issue and to just dispute it if anything came up.

Well, I just received a bill for $600.  There is no way this dent was caused by us.  We were only in the car for an hour and a half and while driving it, we didn't run into anything and nothing fell onto the hood. 

The bill says it took 10.5 hours to fix the car and I'm getting charged for 2.6 days that the car wasn't being used.  I asked them to send me before and after pictures of the car and they said they would send me them if they took any.  Just got a call today demanding payment. Guess I'm not going to be able to see what $600 does to a dent the size of a dime.