I paid $ 125 for transmission fluid and filter change at a Mercedes dealer. This type of service I only have done at a a Mercedes dealer, due to the sensitivity of the transmission.
Also, the trans was obviously replaced under warranty, and obviously defective. But you now seem to have a somewhat new transmission! I'd be happy about that!
As for the kick-down switch, is the switch working properly? Is there a resistance point at the end of the petal? And then does it "click" when pushed further? If not, then your kickdown switch would likely be the culprit, I believe about a $28 part.
Good luck and let us know what's happening!