Bullshit. It takes crazy amount of labor to ship in one day instead of the usual 2-3 days.
It requires employees on shift every night to handle any package immediately. It requires employees to work on Saturday and Sunday to cover week end orders. It requires employees to do longer hours to complete any burst of orders at any time.
What do you think happen to items that are not in the vicinity? They have to be transported overnight across hundreds of miles to a closer distribution center. Again, night and high pressure work.