overcharge and poor customer service

I recently traveled to Gaborone and booked a car through the AVIS website 2 days before travel. The reservation quoted a price including local tax and a charge of 120 BWP for an extra driver. When I arrived in Gaborone I noticed that the amount I was being charged appeared higher. I was now being charged 260 BWP for the extra driver and when I showed the lady at reception my booking confirmation she made a copy and said that she would have it changed. <br> However, after finding my credit card being billed for R500 more than I was originally quoted, I looked up my invoice on the AVIS website and found that I had been charged twice for the extra driver (120 BWP AND 260 BWP). The extra amount seems to come from 'local tax' which I was under the impression was included in the original quote. I emailed AVIS customer service and did not receive a response. I emailed again after 4 days only to this time receive a response stating : 'Please be advised that the total rental amount is correct, kindly note that the additional driver was charged correctly.' from Theodora Msibi. How can it be correct if i am billed twice for the same thing???