I have a carbon Roubaix that I occasionally use for commuting---when I communte with it, I use an aluminum seatpost and the clamp-on Topeak beam rack. I only carry a minimum amount of stuff in the rack bag on those days. The clamp-on rack can apparently tend to swing from side to side unless you screw it down really tight to the seatpost. I was afraid to do that on the carbon seatpost that comes with the Roubaix. You can get a decent aluminum seatpost for a modest amount.

FWIW, I can't really tell any difference in the ride between my cheap aluminum seatpost and the "zertz" carbon seatpost that comes with the Roubaix.