Damaged due to Negligence

I brought my car at FST Service and Suspension in Bloemfontein for service. And when they were done they forgot to put back the covers under the engine and underneath part. Unfortunately it was raining and the water damaged the bumper, broke the sport light. The following day I took my car there to show them that they did not put back cover of which they acknowledge but they refused to take responsibility for the damaged caused by the act of negligence. My car is Mercedes Benz B200.