Alleged chipped/cracked windscreen

Returned vehicle to Tempest car hire.Vehicle was checked & cleared as ALL IN ORDER. Went into Airport building to catch my flight-10 mins later was called back-ran over to Tempest - was told there was a chip and crack on left side of windscreen. Sadly cause I was in a hurry I signed the accident report and ran back to the building. Later when back in Cape Town I phoned & wrote to their customer services re the fact that I don't know where this chip/crack came from cause I never heard anything hit the windscreen. I questioned them that it could've happened (purposely by someone?) during the time I had gone into the building. Bad luck they say \you signed the acceptance form"I denied that this chip/crack happened while I was driving-I AM NOT DEAF I told them! This whole thing has now cost me short of R6000! I tried to say I will sort this out with my insurance or at least pay for the excess!? No deal they said! Now I ask this-their insurance will pay for this windscreen to be fixed right? So they only pay for excess-right? (say R500) therefore they pocket R5500 from me WHY? Friends ALL say the same-NEVER will they use car rentals again because of this sort of saga-they use UBER now"""