If users see the error: “You are not allowed to enter a blank project and cost category” when trying to enter a shipment invoice, there is a simple fix for this. This is a user level setting in Project Accounting so it can behave differently for different users.
The setting is controlled in Project under Setup | User. Select a user and pick Purchasing Invoice. Check Allow blank Project and Cost Category to allow invoicing without requiring a project. A similar setting is found under Purchase Order button to control P.O.’s the same way.