From the video I saw from Amazon about their warehouses, they work like this.

1. The goods are stored in stand-alone cabinets containing multiple slide-out bins. The cabinets are stored in a large section of the warehouse that humans do not normally go into.

2. When an order needs an item stored in bin X of cabinet Y, one of Amazon's robots (a short flat wheeled robot--like a robot vacuum but squarer and beefier) moves to under cabinet Y and lifts it and brings it to the workstation of the worker who is packing the order.

3. When the cabinet arrives at the workstation the system shines a light on bin X. The worker grabs the required item from that bin and packs it into the outgoing order. Repeat for any other bins with items needed from that cabinet. Then the cabinet is taken away (back to storage or I would guess to the next worker who needs something from it).

4. Similar for received items. The system brings the cabinet the item is to be stored in to the worker, shines lights on the bins the worker can put it in, notes which bin the worker put it in, and when done with that worker takes the cabinet away for storage.

So...why not get a bunch of portable toilets mounted on bases that the robots can move (or better designed specifically for Amazon's needs)? When a worker needs a bathroom break they could inform the system and it could bring a toilet to them.

This would also allow the toilets to be brought to a specific area for cleaning rather than having a janitor have to make rounds. The toilets could probably be designed to make it easy to automate much of this.