Unfair Valet Charge

Good Day, Yesterday at 11:30am we returned our rental vehicle to the East London airport branch. The car was in exactly the same condition it was when we collcted the vehicle. When we were done, the inspector told us we needed to go inside to sign a consent to valet form at a charge of R200 over and above the charge for the car. We asked why, and he said that there was too much beach sand in the car, granted we came straight from sand boarding to the airport, and there was some sand in the car, but not near enough sand to warrant a full valet on the car. I have 2 major issues with this, and dont think we should have to pay for the valet. Firstly, the sand that was left in the car would easily be cleaned by a normal car wash that makes use of a vacuum cleaner, i could've cleaned the inside of the car in 5 minutes if i had a vacuum available to me. Secondly, when collecting the vehicle, there was no mention that a valet would be charged if sand was found in the car, which you think would be stated up front in coastal regions, this is not made clear to the customer, and not fair if this is hdden in the fine print. I would like to be contacted by a manager regarding this issue please.