USPS work-share is a program where the U.S. Postal Service offers discounted commercial postage rates to mailers and shipping partners who do part of USPS's work in advance — specifically, sorting, barcoding, and routing mail before injecting it into the postal network at a deeper point (an SCF or NDC) than retail mail enters.
The discount compounds with how much work the partner does. Mail that's been CASS-validated (address-checked against USPS's master file) earns one discount. Mail that's NCOA-processed (run against the National Change of Address registry) earns another. Mail that's barcoded with the Intelligent Mail Barcode (IMb) earns more. Mail that's pre-sorted and drop-shipped to a deeper network point earns the deepest discount.
A direct USPS work-share partner — like L5 — has a contractual relationship with USPS, holds the operational infrastructure (presort equipment, address-validation systems, drop-ship logistics), and passes the resulting commercial-plus postage rates through to its customers. This is fundamentally different from a "discount carrier" that simply resells a UPS or FedEx commercial agreement: work-share rates are USPS rates, not carrier-broker rates.
The structural savings on work-share-eligible mail typically run 20–40% versus USPS retail postage. The bigger your volume and the cleaner your address list, the deeper the discount.