Dollar Rental Car Fraudulant charges and forced insurance Las Vegas Nevada

My daughter rented a vehicle for Dollar Rental car for 6 days.  She was given a quote of $350 for 6 days, which included the under age drivers fee.  Upon retrieving the vehicle, the rental clerk told her that she was required to get the insurance.  My daughter stated that she was already insured via State farm and as well via the Amex card that she was using to secure the rental.  However, the rental was still charged.  Upon returning the vehicle my daughter noted the $750 charge and was told that $400 was used as a deposit to secure the rental and that it would be returned to the card.  It was never returned.  When she contacted Dollar again, someone else stated that the $750 was the total cost of the rental.  Who in their right mind would pay $750 for a 6 day car rental in Las Vegas, when you can get a taxi at a fraction of the cost.  These people saw a young girl with a buisness Amex card and took advantage.   I'm outraged.