Overcharge and no accounting AND a brown car

I rented a vehicle via the Momentum Multiply chekin on the Avis page. The total cost of the rental was reflected as R 2275. I then additionally took the R 40.00 per day windscreen and Tyre cover for 11 days, i.e. R 440.00. Avis then advised me that my rental cost would be R 2926 and further charged R 4500 on my credit card (which was supposed to be the rental cost plus R 1200 - according to the website). To top things off, they gave me a brown car. My previous rental i also did not get my full deposit back (and I understand that I did not give them a full car on return and as such the petrol would be deducted). What bugs me is the fact that they do not account for what they deduct off the deposit - not a statement, not an email or even an sms. Just take what they feel they are entitled to. I've always had positive experiences with Avis and am worried that something is now amis.