Medicine Bags hold the same as a backpack, but it costs a little over 1 plat. The benefit is that it only weighs 0.4 wt instead of 3.0. This is really useful for any characters with low str like int casters. If you carry 8 bags it's 3.2 wt instead of a hefty 24.0 and it is worth the initial investment of roughly 5g more per bag to be able to hold so much more VT.