i had my aluminium seatpost cut. I think you can do it to carbon just like with a fork. the fork needs to be cuts squarely because it needs a flat end for headset loading but for a seatpost it doesn't matter so i think you can just do it yourself without a saw guide. do wrap the place you cut with tape first and cut throught the tape. it stops the carbon from splintering. similarly, cut a a nick on one side first and then cut from the opposite side. this will prevent the final cut tearing the fibre at the end.