A few years ago the Post Office established a package rate for parcels that are tendered to the Post Office from where they will be delivered, providing that they have a USPS barcode, for a very, very low rate. Their carriers are going to every mailbox every day, so, they could offer a pretty good rate.
UPS and other carriers saw this as an opportunity to simply act as the carrier between the shipper and the delivering Post Office. The Post Office isn't delivering these for free, but for a very low rate. Likewise, we are picking the packages up and delivering them to the Post Office for a very discounted rate, compared to delivering them to residences. However, since we seem to lose money on residential deliveries, we are choosing to deliver a bunch of residentials to the Post Office, hopefully, at a resonable profit.
From the ones I deliver to Post Offices in a very rural area, I can assure you, UPS makes a lot more money on these packages than they do when I hit the gravel.