Price quoted not the same as final payment

We reserved a car for our children who visited from New Zeeland and gave our Credit card number with the understanding that the rest of the rental would be paid by the children , we were quoted R 3421 and the full amount was taken from our credit card and on top of that they paid R 5107 . This means that we were overcharged , Firstly I took on my bank and eventually we found that travelstart was the culprit . I sent numerous emails to their [Email Removed] email address for the attention of a cerain person , I eventually got a relply form Nazley upon which I asked her to call me but nothing happened. I still feel that we have been overcharged and I do not get any satisfaction from Travelstart.