Okay, so maybe it was a bug working in our favor which is another theory I had.
Forgive my lack of knowledge, but if I am understanding this, we could add a map and just put 2 in there for the ship days element in the file? This is not something our partner is going to change.
I found this mapping note in the EDI tran tech ref guide and it seems if our partner leaves it blank then it will use a field on the customer record which could also work.
You can leave it blank “” and Epicor should map it by reading the Days Out I think from Demand Settings in Customer/ShipTo… more than likely you are not leaving it blank, but rather a 0 or Todays Date.
Or figure out the date to go in there. and its YYYYMMDD