There are about 10 slots in their largest box, and they are charging $4 per slot. The average package will occupy the slot for a day before it is picked up. This gives a max revenue of $40/day/box, or $1200/month/box. Slash that figure in half because it is impossible to have all slots occupied simultaneously all the time, and you get $600/month/box.
If they can turn this into a profitable business, they are truly visionaries that see what others cannot.