[mothered]

But, I still don't get why a hypothetical CCTV system in their warehouse shouldn't make my SE fail.

That's because the CCTV Is taking footage of the boxes being picked & packed by the storemen, and not what's Inside.

That Is, they receive the picking slip with your order, head over to the location of the racking where the boxes are stored (In the warehouse) and pick & pack them In readiness for dispatch.