Sorry, but there is no option to determine what cost is used to pre-populate the Unit Cost for new Orders - it will be the Most Recent Cost unless your items use the Standard Costing method. The way that the Avg Unit Cost gets applied to items on the invoice is to run Day End Processing, which is the usual procedure.

You can run Day End as often as you wish - it doesn't have to be at the end of the day. Of course, you have to have people stop entering orders, so this may not be practical.

Good job on the GridView view! You are correctly dealing with the 'qty shipped not costed' to get the qty as if Day End in OE was performed.