From the USPS:

The USPS deployed the USPS Returns platform on January 27, 2019. This automated return service is replacing the Merchandise Return Service (MRS) and Scan Based Payment (SBP) legacy systems. We are allowing existing customers to migrate to the automated returns process and new customers can only enroll in USPS Returns.  Current MRS and SBP customers must migrate to USPS Returns by August 28, 2020 per Federal Register Vol. 85, No. 37.

The Postal Service is utilizing upgraded mail processing equipment to capture near real-time data on package dimensions, weight, mail class or product, and other attributes.  This technology has allowed the Postal Service to streamline the processes for the identification and postage assessment of each return package, enabling account holders to pay the postage for their returns electronically.  This improved functionality will significantly reduce the need to manually weigh and invoice returns or to estimate postage via sampling under the Postage Due Weight Averaging Program for MRS packages, and will eliminate the scan-based payment process currently used with USPS Returns services.

Current MRS and SBP customers may initiate their migration and open a USPS Returns account by contacting the USPS Mailing & Shipping Solutions Center (MSSC) at 1-877-672-0007.  Business hours for the MSSC are Monday-Friday from 7:00 AM-7:00 PM Central.