Bidvest charge for minor damage despite full insurance

After being my favourite car rental Bidvest has blown it. After my bakkie was written off in an accident I rented from Bidvest for 3 months. As an NPO I can't afford to take risks so I took out insurance plus a waiver against excess. Whilst driving the car a tyre blew out and the car pulled to the left (there must have been an undiagnosed fault with the car as they are designed to move in a straight line when a tyre punctures). I was going quite slowly at the time but I just grazed the kerb causing a couple of scratches on the left front bumper. They charged me just over R3500 for the damage. Apart from the fact that I was insured to the hilt, the damage was minor and I can't believe it cost that much to put right