Unfortunately...there aren't too many decent tire shops around the Hartford area and a few I've dealt with had no clue about the special weights used on MB wheels...not to mention they employed monkeys who had problems putting directional tires on correctly.
Though paying more at a dealer, unless you have a super indy; its your (at least my) best bet to have a dealer wrench stuff on the car...even to mount and balance a tire. I myself used to mount and dismount tires back in the day...its not hard like you say but some folks just don't give a crud when they do 50+ rims a day.