It really is OK to ship as a piece of baggage. However, just to be on the safe side do the following.
Leave the box untaped so you can open it and show the check-in personnel that there is no fuel "sloshing" around. Bring tape to secure it after it has been "inspected".
If that fails for some reason because someone is not sure about accepting it, please request that they kindly phone UAL cargo personnel who can then (with 1-2 questions) determine it is safe to fly.