Originally Posted by jsharr
Probably the 2 mm difference does not matter. If any flex occurs, the seatpost below the shim will still contact the wall of the seat tube and add some support. I think as long as your shimmed seat post is inserted past the minimum insertion mark you are safe. Part of the idea may be to not have a lot of exposed seat post, making a long lever arm with lots more force on the seat tube/top tube, seat stay area. I am not a mechanical engineer and I have not stayed in a Holiday Inn Express, so these are just my guesses.
First, its 3mm difference not 2mm, 1.5mm on each side, and no way would you want the post to move that much while in the frame. If its an aluminum frame, the situation is even worse. Just get the right seatpost.