Coupla comments. I think you actually got a pretty darn good price for having a shop do this work. Other folks have posted much higher sums of money for the same work. Of course, the million dollar question is whether you needed this work.
MB publishes two thickness figures for brake rotors. The minimum thicknes is the thinnest it can ever be allowed to wear. Below this point it is unsafe to use. A maintenance or service limit is also specified. This slightly greater thickness specifies the minimum permitted thickness at the time brake pads are changed. Below the service limit, the rotor will wear below the minimum thickness before the next brake pad change. The only way to determine whether your rotors were below this value is to measure them. The presence of a ridge means nothing beyond indicating the rotor is no longer brand new.
As well, the shop should have changed the brake fluid with this service, not merely topped it up. MB specifies brake fluid is to be changed at least every other year. Doing so preserves your safety and extends the life of brake components such as the master cylinder and calipers.
My opinion: the shop gave you a good price, but is very sloppy. If they didn't measure the rotors, didn't change the brake fluid, and did spill brake fluid on your paint they aren't good enough to work on my cars.