Avis ripoff and atrocious customer service.

I had a 4 day rental, charged at R3100. On the last day I needed to extend the rental by up to a day. I called the company and was advised that the call was being recorded and that the charge would be R900. I accepted the change, but when I was billed Avis charged an additional R3000. I made calls to the national call centre and was told I would have to call the branch. The branch does not take phonecalls. I called customer service again and a complaint was logged, and I was promised they would respond within a few days. They never responded. I emailed Avis to query the charges. A customer service agent promised to respond within 5 days. There was never a response. Numerous attempts to email subsequently and request escalation have been met with promises of escalation - no response has been forthcoming. It is clear that the service agent could not even be bothered o read my emails.