I was worried about that too. I didn't know exactly how they worked at first and someone had to walk me through it. I would I would be getting multiple slips for each armor. I was like "that's not saving space?!"

But I have saved so much room y using them. I no longer have to mule when changing from melee to mage jobs. Porter moogles were one of the best additions in game.

Although I am not sure about what would happen if timed out while trying store or retrieve armor with a slip. Best wait for a Dev to be 100% sure.