These third-party sorters collect mail from mailers, sort and apply barcodes, and deliver the mail to the Postal Service for a portion of the discount.
3rd party sorters collect mail, sort it, apply barcodes then transfer mail to the USPS for a lower cost.
The Decline in  Evidence from Household Surveys, Brookings Papers on Economic Activity (1:1991), William C. Brainard and George L. Perry, eds.