Pro forma invoice for non-billable shipments

Some shipments are never invoiced to a customer. Example: Shipment to a subcontractor or manual warehouse transactions such as Sales(Manual). However, such a shipment may also require a pro forma or customs invoice documentation. A pro forma invoice can be created for such non-billable shipment. In Invoicing, these pro forma billable lines are handled with source type set as 'Shipment'.