Unfair extra charge

Due to the flysafair strike, my flight times changed. I picked up the car at 3pm, and was told that the agent would give us an extra hour, so we could return it at 4pm. My flight was at 8pm that night, so I was under the impression that it was out of kindness that they gave me an extra hour, so that we would only have to wait 3 hours as opposed to 4 hours at the airport. NOWHERE was it mentioned that I would be charged an entire day for the extra hour. Had I known, I either would have returned the car at 3pm, OR spent longer with friends and returned the car at 7pm. I would be an idiot to take an extra hour for a full day charge!!! I am extremely upset about the extra charge without any communication from the company. I have tried explaining to them, and they are not interested.