My husband commutes on his carbon trek5500 with a carradice sqr slim bag, but he changed out the carbon seatpost for a thompson aluminum one (also light weight). He also removes the bag clamp (3 oz) for club riding. Another friend does the same, but has two seatpost/saddles, one that is aluminum for carrying the carradice bag and one that is carbon for fast club rides, and he just switches them. I also commute with this bag (just google carradice sqr slim), but keep it always on my commuter bike. It is the lightest option I know for commuting, and will hold a laptop plus some clothes, lunch, etc.