VERIFIED SOLUTION i

Information Where do the USPS Postage rates that MailStream Plus uses come from

MailStream Plus™- all versions, all platforms
Postage rates for all types of mailings and mail pieces can be found on the USPS's 'Notice 123 Price List' located here- 
https://pe.usps.com/cpim/ftp/manuals/dmm300/Notice123.pdf
(link current as of Feb. 22, 2019)
UPDATED:  March 11, 2019