Found this on Fatwallet thanks to poster Jesternl and did not find it here through a search. Apologies if it is here and I can't find it. This link worked for me and I do have discounted FedEx shipping, but not at the point that disallows miles earning apparently.

With the exciting new relationship between FedEx and Delta, you will now earn 500 miles for your first and fourth eligible shipments with FedEx. Plus, you will earn 1 mile for every dollar spent on FedEx shipping services. It's that simple.
We value our all of our customers and regret that we cannot offer the Delta SkyMiles program to our customers who already have a discount of more than 26% on their shipping.

https://www.fedex.com/fcl/ALL?enrollmentid=bs12525310