Charged for a valet because of a few sand grains

We normally go with AVIS. But never again. I really think AVIS could have done better. Yes, a person signs the terms and conditions and Yes a person does not read each and every line item in the terms and conditions. However, if we were informed about waivers, why could we not be informed about the VALET. Only when we were dropping the car off, did the guy checking the car in, advised saying we are going to be charged an amount for cleaning because of the few grains of sands were the children were sitting. When I enquired about it, AVIS said, you signed that you knew about this. Why can they tell you about the waiver (because it is a huge amount), but why can they not tell you about the valet (+- R440.00). Also, surely each and every car is getting vacuumed before passed onto the next client. I really do think this is is not fair, because in my mind a valet means washing the car inside and out. Ridiculous.