I'll also add that the Add Item Slot actually adds usable item slots (99 Bag Slots doesn't really work).
However, the item slots added in this way do not remain, so if you're using this, be sure to not leave any items in slots you don't actually have, or you'll likely lose them (I haven't pressure-tested this so I might be wrong but better safe than sorry in this case). It is useful to add temporary slots in the case of gathering a bunch of resources for merge shops or for gathering quests and so on...
Still kinda wish there was a way to add these things permanently...ah well...