Go to a different shop, and never go back. That shop doesn't know what it is talking about. Use a place which specializes in Mercedes to change the transmission fluid & filter, then see how it works.
FYI, you can get a replacement transmission installed at the dealer for ~$2500, including labor and such. It will carry a 4 year/50000 mile warranty. The guys you went to are a bunch of jokers & ripoff artists.