Avis Southern Africa poor service

I rented a vehicle from Avis Port Elizabeth end of June. Upon arrival no one was available to check the vehicle and I was told to do the whatsapp prompted vehicle check. This prompts you with what you need to check and then gives a report before departure. I trusted this was sufficient. On returning the vehicle they noted a spare wheel was missing (I did not check for this and also not prompted by the Avis bot to check this) Then I was charged R6500 for this spare wheel. The customer service has been appalling to say the least and no communication except the poor customer service agents in the middle. How do I contact a senior management person as I am so disappointed by Avis Southern Africa.