There's no requirement to use a carbon seat post. I use an Al seat post and bars on a carbon frame.
If you mount a saddlebag properly there is very little cantilevering or motion, but it's still nice to keep the weight to a minimum. My Carradice Camper fully loaded weighs 12lbs and is supported by loops on the saddle and a strap around the seat post. I have done thousands of miles with this setup and never had an issue.
24L capacity in one of those things, I wounder how stable it is compared to one of the revelate designs viscacha bags