I don’t know of any “official” reason why the UPS discount is there and sometimes it isn’t. My own observations are that it seems to depend on where the package is going. Yesterday, I had a shipment going to Utah, no UPS discount, also had another shipment going to Minnesota, which had the UPS discounted rate. Both shipments were standard, not expedited, and both orders were made within 15 minutes of each other. Carriers have routes that are like streams to all parts of the nation, some streams are probably more efficient than others, these more “efficient” streams are probably the ones getting the UPS discount. Just my opinion and thoughts on the subject. Is Amazon trying to move us away from USPS? Maybe. I think they just want sellers to get their stuff to the customer quicker. If they can lure you away from using Media Mail, for example, by enticing you an “expedited” service with another carrier for a price that is competitive with the cost of USPS media mail, then Amazon probably expects us to use it and enhance the customers buying experience. I only recommend using UPS if you have your own UPS account linked in to Amz’s Buy Shipping.