We also require an order shipped date. We may have orders placed over the weekend. These are then downloaded on Monday and are shipped on Tuesday. We require a Ship Date on the invoice so that the customer can see the time period for dispatch and for us to be able to Batch Ship orders.
Not sure why the field is not already a standard field.
Very useful to be able to run a report by order Shipped Date too so you can determine the volume shipped per day etc.
I am sure this is very easy to add.