Shocking client service

I rented a vehicle at the Tygervalley branch and paid for 3 days. After 1 day I didn't need the vehicle anymore and decided yo return the vehicle in order to save money. When I returned the vehicle, the Avis agent confirmed that I will only pay for 1 day. When I contacted the call centre, they told me that according to the T's and C's I have to pay for 3 days. I never read the T's and C's, but I feel it was the responsibilty of the agent to inform me when I returned the car. I've received no communication from Avis regarding this matter and will never ever make use of their services again.