Incorrect invoice and they refuse to correct

I hired car from Tempest Kramerville for 30 days. When i extended the last 5 days the guy from the call centre told me that their invoices run for 30 days so i have to bring back the car on the 12th. When i picked up the car I told them i had an additional driver and they said ok. After i returned the car i got a call from Thamsanqa from Eurocar who claimed that i was owing R4424 for 2 additional drivers that i know nothing about and extra kilometres driven. When i asked him to do a physical calculation of how many kilometres i drove it turned out i had driven less than the free kilometres allocated to me. He then said i should call the branch, i called the branch and they explained that when they closed the invoice at 30 days they used estimate so they have to correct the invoice but Thamsanqa keeps insisting that i pay the full amount!!!!! But the kilometres are still within the free kilometres!!!!