About 2-3 years ago the USPS started deploying an automated parcel sorting machine (APPS). That changed the way parcels are handled. They used to be handled by a mix of machinery and personnel. Now, once the clerk at the PO puts the parcel in the wiretainer to ship, it goes from there to the nearest distribution facility with an APPS, and from there it gets sorted almost exclusively by machine. The one exception to this is live animals, which are manually handled from door to door.