Multiple and Partial Shipments
            In this scenario, your Customer has ordered several types of products, in various quantities.
            You have prepared an Invoice with a list of Products ordered.
            And you start shipping products as soon as they are ready, i.e. it takes several shipments to deliver all the ordered products. 
            You can 
create a Packing Slip from the Invoice. When a shipment is ready for delivery, edit the Shipped Quantities for each Product included
            in the Shipment, and print out the Packing Slip.
            
            
Tracking Previous Shipments:
            Each Product in the Packing Slip includes a "Previously Shipped Quantity" field which keeps track of the number of items already shipped.
            After the first shipment, whenever you update the Shipped Quantity of a Product in the Packing Slip, the Previously Shipped Quantity field is automatically incremented by the previous quantity shipped.
            (Note: this is subject to your prior confirmation via a popup message).