Faulty charges and refusal to respond

I rented a car for two days in Namibia which was supposed to cost NAD1000 per day maximum. When I returned the car, it checked in as being 100%, and the agent said they would send me a final invoice. The following day two charges came off my card for a total amount of NAD3000 per day (3 times the price!) and they never sent a bill reflecting the reason for these charges. When I tried to get through to Customer Care, they say they only respond in 10 days. In the meantime I am travelling and have a deficit on my credit card. This is effectively ***** - levying charges on a card which have not been agreed to.