Over charging over contracted quote

After 50 years of marriage, my wife and I decided to visit South Africa starting in Cape Town. We booked a vehicle through Airport Car Rentals for a total price of R4073.35 and paid a deposit of R693.55. This left an amount of R3380.00 to be paid. We also took out a comprehensive insurance police for AU$141.48 . Upon returning the car, they charged us R4150.20 being R770.00 more that the agreed amount. The additions being for 'Hail Damage' , 'Tyre Waiver', 'Glass Waiver' and Etoll. They claimed that when I signed for the car I had agreed to these deductions! However this was not pointed out to me and surely my insurance policy would have covered any damage? I have contacted them a number of times but they refuse to refund me. I have advised all my friends not to do any business with this company.