Damage to rental cars + consequences

Hello, I drove a Sixt car on the highway over a piece of truck tires. There was a slight, superficial scratch in the driver’s door. Otherwise no damage. The scratch was barely visible under the dirt. I still reported it honestly as I am. Since the scratch is really superficial (8cm long), I had hoped for some coolness of Sixt. Now the bill has come. Repair 630 Euro plus expert costs. . I find it outrageous and disproportionate to paint the door completely because of this scratch. Would I be pleased with ideas on how to equip my complaint with good arguments. What possibilities are there, for example, to remove light scratches without painting?