Avis overcharged R5400, still no refund after 3 months!

We hired a car from Avis over the holiday period. We used the car well so we were quoted a valet fee of R599.99 at the end of the rental. The car was dirty, so we accepted this. Then however, my Credit Card was charged R5999.90 for the valet. The same amount was part of Avis final account. When I confronted them about this they admitted that it was a mistake, and that they would refund the money. It is now three months later and I still did not receive this refund. Despite two promises from the Customer Service Department that the refund was processed, which evidently was not true. I begin to believe that this was done on purpose because my card was an international card. Maybe they hoped that we would drop the case because of the distance. Anyway, whether by design (*****!!!) or ineptitude this issue is shocking. Beware.