Billing of account without my permission

I had a courtesy car from Avis through my insurance because my car went for repairs on the 5th January 2016 and my car was repaired and collected on the 22nd January 2016.I was then contacted by Avis that I should then bring back the car on the 22nd Jan and failure to do that I will then be liable for payment each day after the delivery date.I then agreed over the telephone on this term and that I will pay for additional.On the 25th January I got an sms from Avis saying I have an overdue amount of R605.00.I was planning to return the car on the 27th January at 8h00am but this morning an amount of R2491.73 was reserved by Avis without my permission and this is on top of the R 1200 amount I made for the courtesy car.I have a big problem with this especially when I didnt give permission for Avis to reserve this amount.They called me to say they are going to collect the car,which they are more than welcome to do so.However I dont appreciate the way they deal with customers.