While most stokers like a suspension seatpost, they are not always 'needed' as Team Ti700 indicated.
Stoker Kay has ridden for over 31 years on tandems and has not had suspension posts on any of our personal twicers. While we have test ridden many tandems, a few with suspension posts, the only suspension she liked was the carbon fiber Softride beamer.
It does help that captain communicates the unavoidable bumps/rough road.
If your stoker really wants to try riding with a standard post, that option is relatively cheap; but hang on to the suspension post, just in case she changes her mind!
We do have a rear rack with a trunk bag on it in cooler weather or on trips, to carry extra clothing/what ever. Also, we carry a plastic container in a water bottle cage for extra tools/tube.
There's all sorts of ways to carry 'stuff'!
Pedal on TWOgether!
Rudy and Kay/zonatandem