Charged incorrectly for damages.

I recently hired a car from Avis and specifically the AVIS Cape Town International branch. I rented a Hyundai Tucson and on collecting the car I was just given the key and shown where it was parked, without any employee accompanying me to the vehicle to check. I went around the vehicle and checked everything and all seemed fine. I did notice at the time that there was no backboard in the vehicle as I was packing in the luggage but I assumed that the vehicle did not come with a backboard or that it was already accounted for. On returning the vehicle 7 days later, I made a note on the return sheet that there was no backboard received when I picked up the car. I was totally shocked and utterly disgusted when I received an sms and an invoice of being charged accident damages of 7000 rands + which I assume is for the backboard. This is shocking and really need this escalated. I will not be paying for something that I should not be charged against me. I need this investigated immediately and someone to contact me regarding this.