Avis rental deposit return

My wife rented a car from Avis from 22/01/2019 till 31/01/2019. The rental price was R 2 978.12 plus an additional deposit. After the rental period expired she extended it three more times. When you look at the website the daily rate for the Yaris she drove is just over R 500 but each time there was a debit taken from her account of more than R 900. When we queried it Avis just said that it was extra deposit. When we returned the car it was cleaner than when we picked it up, however we did not fill the tank again and we where aware of that extra charge. The thing is that when we received our deposit refund it was for the original deposit paid on 22/01/2019 less the fuel. What about the deposits taken with extension on 30/01, 01/02 & 06/02? When I contacted Avis in Paarl they where very helpfull up to the point where they could not answer that question for me.