Stop wasting your time with these morons. Get a new cap on there, gunk the engine down, and have a better tire shop balance the tires. Someone with a Hunter's GSP9700 road force balancer.
If its a crappy balance, $40-$60 and you never have to see them again. If the tires are out of round than go back. If you bark a bit you might get the balance at the other shop covered.
Its not worth wasting time and effort on morons. Lawyer up, over what a $60 balance, come on.
You will learn that its not worth spending a great deal of effort on morons, because nothing you are able to do will change a thing.
Your mistake was taking a MB to a shop that does Toyota's and Chevy's. People that have those cars are just happy to have tread on the tires. Lesson learned, I learned a similer leason, for the price of two front brake rotors, and a balance. Cheaper than school...