Holding me responsible after signing a car off and debiting my credit card twice without my permission

Mutual and Federal booked a rental car for me from Avis Budget as my car had to go to the panel beater for repairs. I requested that the car from Avis would be delivered where I handed in my car (Auto Magic Menlyn Motor City) The Avis car was delivered on 14 July 2017 and I was requested to pay a deposit. I provided them with my credit card details for the deposit. On 19 July 2017, I was informed that my car was ready to be picked up and I requested Avis to collect the rental car at the panel beater. They collected the car and I signed it off. At about 18:00 they called to say they discovered a chip on the windscreen. I told them that I knew nothing about it and they also did not discover this when they inspected and collected the car. They invited me to have a look at the car which I did the next morning. They then showed me chip right on the far edge of the windscreen that no one could have noticed.

* It could have been there when I received the car * It could have happened when they drove the car from the panel beater in Menlyn to the depo in Menlo Park

They never came back to me about the case and only debited my credit card, without my permission, for R 3,304,03, dated the same date I rented the car, NOT the day I returned it. I never received an invoice or any letter from them.

Now, nearly 3 months later, I received a call from them, informing me that they will late debit me with R 450.00 excl vat, to my credit card, as the invoice was higher than the amount quoted.! I told them I refuse to pay the amount but they had already debited my credit card with R 524, again without my permission! Questions Can I be held responsible for damages to a car after I had signed it off? Can a firm debit my credit card without my permission? Can a firm come to me 3 months after they had billed me with a further amount?