Orchid View Extender can do this:

https://www.orchid.systems/product/extender 

Or you could use a required Optional Field on the order header instead of the Ship Via field... but the duplication may look a little odd to the users.

I would agree with using Orchid's Extender. 

Another option is "Alerts like": put a SQL trigger on the Order Header field to sent out an email (using SQL Mail) when a record is added/updated that is missing the Ship Via field.  This is after the fact but it will be fairly quickly after the fact. 

Thanks for the great ideas and solutions. For now I've implemented SQL triggers with email notifications. It sends an email to the specific user who posted the Order missing the required field. Side note, does anyone know where the Windows Username field is stored for Users in Sage (ie. What db table)?

Users.ism
It's a file in the Site folder, not in the db.

You could look at view AS0003 and dump that information into a SQL table.