Receiving an existing item that has a different cost: how to amortize the two costs automatically?

When we receive a purchase order that has items which already exist in our inventory, yet perhaps have different purchase costs than those already on hand, how do we get the costs to amortize?

In other words, if I have 3 of an item that cost me $2.00 each, and receive 2 more that cost me $3.00 each, is there a way to get the POS to automatically update the "Cost" on the item's "General" tab to show the amortized cost of $2.40 for all five items?

I'm fairly certain there is a check box I have simply missed. By the way, we have tried the pull-down on the Purchase order that switches from Quantity to Value to Manual (according to the Help, the Value option should do what I want, but is not).

Sincerely,
Robert Placer
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