Avis ORT unauthorized charge/poor service

Avis at OR Tambo Airport added an additional charge onto my bill without authorization, without even disclosing it to me or asking me about it. I've been a regular customer for a couple of years and this charge was never included previously. Had it ever been mentioned, I would've declined it anyway. Earlier this year I was charged an extra amount for a small scratch on a hub cap, because it wasn't identified on the pre-rental sheet. I'd not caused it and I had to fight with Avis to get it reversed. Last week when I collected a car, there were no markings on the pre-rental sheet, but I found two prominent scratches which I had to find an Avis representative to note on the documentation. This happens fairly regularly, plus it's semi-dark where Avis parks their cars. There's never anyone around to look at the car with a renter. Four days since I spoke to Avis and sent emails to the relevant departments, but no response from them.