Adding Overhead Cost to purchase order at receiving into stores

Hi,
We have a a problem with adding overhead cost to a shipment

We are using Average Cost , and we created other cost type AVGRATE to be able adjust item cost.
for example. the overhead subelement will be used to add transportation cost to the shipment
For Item A at organization X, PO1 The purchase Order Price is 10$ and the transportation price is 2$ , the user will enter a value of 2$ to the subelement transportation in the AVGRATE, while receiving into stores the cost will = Purchase Price (10$) + Overhead (2$) = 12$

in the same time other PO2 for the same item will be received but this PO doesn't require overhead cost - No Transportation Cost -