The Ship By Date field found in ShipStation was created for our Amazon integration, and it's currently only used by our Amazon and Walmart integrations. You can also set the Ship By date manually or via Automation Rule when orders are imported into ShipStation.
If you'd like to see this field added for our eBay integration, we'd appreciate if you'd vote for the feature on the existing thread about this on our Ideas forums, which our Product team uses when evaluating new features and changes we can make to ShipStation.
