Charged for a Guarantee Claim

Rented a brand new Toyota from Hertz (780 km on the clock). I caused no damage, I returned the car with a full fuel tank. During the rental a small body part (tow cover) fell off. That part is literally the size of a matchbox, probably costs R 20. I have been charged R 950. Why have I been charged at all? On a car that new a body part falling off is covered by guarantee. I guess Hertz enjoys the double benefit of claiming from Toyota under guarantee as well as charging customers. I have been trying to get Hertz to refund the amount, but they take weeks to respond and then it is just the standard "You damaged our car - you must pay". I will not be renting from Hertz in future, and I will probably reconsider buying a Toyota as well.