If you look at the mass mailing options that the USPS markets on their website, they ask you to basically buy a whole delivery route at a time.
There's no routing and sorting they have to do. The carrier just picks up a stack of letters at the start of the shift, and drops one in every mailbox.
The labor cost that the USPS has to put in for a mass mailing is a fraction of what it has to do for normal mailing.