Auto Tran Fluid Overfill???

Took my car to Jiffy Lube for oil/filter (normally go to a Euro shop, was in a hurry). They mentioned the usual oil and tranny leaks and sent me on my way. I haven't checked the tranny fluid level since I acquired the car last fall. I searched this site and my owners manual, both of which confirm that I should be reading a fluid level of 10mm below the min line (I assume it's the lines at the very end of the stick) when engine is cold. I go to check the fluid on my cold engine (not driven in the last 3.5 hours), and discover a tranny flud level 20mm OVER the max line. Now from what I have read on this site, overfilling is a very bad thing. I don't think the Jiffy gang did this (heck, I think this was there first Mercedes to change the oil, much less know where the tranny check is), and therefore I am assuming I've been driving around like this for some time. Am I missing something? Fluid is a medium red, doesn't appear dirty. My shifts are abrupt under normal acceleration, made less abrupt if I slightly lift off the gas right before the auto's next gear shift.

I plan to do the more accurate check in the morning with the engine idling after a few minutes. But how quickly do I need to drain off the potential excess, if this is truly an overfill condition? I'll be pretty unhappy if the Euro shop from my previous fluid service is to blame

Thanks.