Client feeling cheated by Avis Car Rental

I Am having the worst experience with Avis Car Rental at OR Thambo international, I rented a vehicle from them on the 22-26th of November.upon taking the car a quick check took place between myself and one of the employees he ticked off a few things that were wrong with the car and he handed me a copy and i went about my plans for those few days.I took the car back on time and another check was conducted by myself and another employee he showed me a few dents i then explained to him that those were there when I took the car i had the copy to prove it.He then asked for my copy which was odd since they had a duplicate I Obliged thinking he would hand it over when he as finished but he never gave it back.he then said all was well and that I could leave as I wouldl get an sms of my invoice.Then later that day i received a text massage and it read(kindly note that on our second inspection we found damage on a vehicle you returned to us.please contact us on *** Ref.No ***) then they called me using this contact *** we experienced a network problem I then tried to call both numbers on Wednesday to no avail then on Thursday i went back to them to find it about this issue they asked for my reference i gave them the lady took about 15 min whilst i waited for her and she told me i damaged one of the tires she said they found a nail i then requested to see the vehicle and the nail since the check upon my return was already done.i asked which wheel was damaged on the car she could not give me an answer and when I asked to see the car she told me some story about it not being there anymore but they had taken it to their head office.she rudly said I just should wait for the invoice since they were going to withhold my deposit and that I should just pay for the damage.Today at 09:48am I receiced yet another sms from them saying(please settle your Avis Acc REF.No E*** of R3,088.43 within 7 days or contact Manase Sebate on *** for any queries.Nedbank *** code 196140)again i tried calling the number it was unavailable more than 3 times.So is this how a lot of their clients get treated and cheated out of their money? is this their way of getting extra cash into their pockets or is this really how their system works.my contact details are ***.