Why do you think eBay isn't giving shipping discounts any more? There has never been a discount for Media Mail. And the eBay label rate for Parcel Select Ground looks like there's no discount, but that's because the retail equivalent has a different Name: Retail Ground, and it does cost more than Parcel Select Ground. (Both services used to be known as Parcel Post.)
For other services, eBay DOES provide a shipping discount, in the form (as always) of charging the published Commercial online rate rather than the retail rate that you'd pay at the PO counter.
In fact, recently eBay announced that sellers will be paying even less than the published online rate for some Priority Mail services, but the announcement didn't specify which weights/zones, or how much less: "Sellers using eBay Labels will save even more on Priority Mail labels for select weights and zones, with new rates up to $1.02 lower than Commercial Pricing. "
https://community.ebay.com/t5/Announcements/New-lower-rates-for-USPS-Priority-Mail-labels-effective/...
