Change of headlights after rockfall

Hello, in my 190’s, a headlight was smashed by a stone. I called my insurance company and they took up the damage and gave me a damage number. Now I drove to the Mercedes Benz branch and ordered the damage to be repaired there. There they put in a new headlight. I should only sign a cession declaration and specify the damage number. Today I was there again. d the boss came with a letter from my insurance company, which tells there that the headlight glass is to be obtained individually and therefore only a partial amount is paid. Now I should pay the difference of 300 euros itself. Is it not up to the workshop to know that one gets the glass individually.Why do they install an entire headlight, if I have informed them that I want to settle the damage over partial cascade.