I think UPS is handling all the air delivery for the USPS then dropping them off at the respective postal depots....
around here a amazon prime van (a dodge promaster) is seen pulling up to our post office at about 4 am. They dump the whole van worth at the post office and they sort and deliver.
USPS is doing last mile delivery. The argument by the PO is “we are going to these addresses anyways” and for the most part they are right. someone has to bring the junk mail.
but my carrier says dealing with amazon makes his route longer. his 4 door jeep is packed with amazon packages. They recently made changes, before they would have to come up my drive and leave the package at my door, of course, this takes time (my driveway is not short by any means) now, if I didn’t have a rather expensive package box by my mailbox they would just leave a “sorry we missed you” slip and expect me to drive to town and get the package. essentially doing their job for them.