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 take longer than to simply use the postal service sorting. 
mnli hypothesis: 3rd party sorters take longer than to simply use the postal service sorting.  premise: 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.