1

Suppose that we had shipped the orders and set Order Status as Shipped. But, what if the shipment doesn’t arrive to the customer due to a problem in customs clearance? So, Shipped status only shows that the items are shipped but not the results of the shipment. So, we need Completed as a way to differentiate the result. Furthermore, we can filter orders according to Shipped status to find the problematic shipments which didn’t change into Completed status after waiting for a long time.

2

It depends on what kind of scenario we will follow for our store. For example, we can generate the invoice before the payment and send the customer email address to inform about where to make the payment, for example in the case of check/money order.