In general, it's better to use the browser based online planner for pre-cruise purchases. Usually that's done on a bigger screen where you can see a bit more of what you doing/buying and usually works better than the app.
The 24 hour package is NOT available in the planner - only on board. It is typically about $3-4 more than the on-board full sailing per day rate. Voom package on board pricing is usually fairly close across the fleet (within $1-2).
For all planner or on board purchases which have one, there's a mandatory 18% grats added.
For the cabin, you can add the $18/day PP grats before the cruise or it will be added automatically on board - those can be removed on board.