There is no rental contract until you pick up the car, so I don't think you will have much luck going for any regulatory agency to back it up.

It would help if you named the location and the fees, I know I have never in hundreds of rentals with Hertz had it be any different then the reservation unless I upgraded the car or something.

I also think it probably would have been better handled when you were picking up the car and not dealing with customer service later.

Plus all of my reservations say something like "Additional fees or surcharges may be applied at time of rental." Like I said, I don't believe I've ever seen it happen, but they have pretty much left it wide open they could add in fees, and disclosed on the contract the fees when you took the car.