I was a "package handler" at UPS for 2 years in college. There were about 7 or 8 pup-trailers backed up to the conveyor belt, each going to a different distribution center. The guys on the line are sorting these by sight using zip codes and throwing them onto the rollers. Since it's part time night work, most of the guys have already put in 8 hours that day at their first job. Or they farm, and do this for health insurance. Try keeping a tired monkey with a brain capable of calculus focused on zip codes for 5 hours. He'll mispick a few packages in every thousand, and off they go to Omaha, Oklahoma City, Chicago, Denver, Sioux Falls, etc. Where the process will be reapeted (maybe twice) before it lands on a package car to your doorstep.
Blue Bunny Ice Cream out of Le Mars, Iowa used to be a UPS client. Until one day they shipped a carton of ice cream next-day-air. It got picked to the wrong trailer; it never made it to New York City, where a studio, a director, and talent were already booked to shoot a television ad. No ice cream, no ad. A very expensive mistake, for which they switched to Fed-Ex (where a different group of tired guys will doubtless mis-sort packages to whole new cities!)