As others have said...a seatpost mounted rack won't work on a carbon seatpost and doesn't hold much weight.
I have Trek racks that I got from my LBS. They work great....they have a variety of styles to fit a variety of bikes, the racks hold a lot of weight (rated for 50 pounds, but who knows if I ever really put that much on them), are designed to hold Trek/Bontrager bags that snap-on-snap-off via their quick-release mechanism, but also accept any body else's bag.
I'd recommend them....