Did this this past weekend and the result is dead silent!
Since my local hardware store didn't have any felt tape, I ended up getting sticky velcro and used the "softer" side, obviously. Worked like a charm! Also used some window sealing foam.
I used the felt velcro along the rear vent (don't forget the sides!), and the adhesive foam wherever I felt a need. Upon disassembling the rear deck, I noticed a lot of noise coming from the overhead liner where the 3rd brake light is. I took that out, and foamed as much as I could there as well. No more squeak, rattle coming from that!
Minor adjustment from the OP - I have a 2011 LCI e90, and the childseat locks are quite different. You cannot "flip" them up to reveal the torx screw (T40). I had to put my arm underneath the rear panel and push the clips off from underneath.
Very simple DIY - definitely worth doing yourself if you have the time and do not like the idea of having the dealer fiddling around, prying off panels, with potential to break something
EDIT: And for those with the Rear Sun Shade option - there are 3 additional screws located in the trunk (underside of rear deck lid). Left, Middle, and Right. Remove those as well. Easy to spot.