Don't understand how you did it, not clear enough on your elaborations.
What usually you should do, assuming that you have an Invoice of Freight Cost of USD500
And previously you have a Purchase Receipt of an item receiving 500 quantity at USD 300 each
When you selected Charge Item in line item, select "Line" > "Item Charge Assignment",
Go to "Actions" tab, Get the related lines that you wish to allocate this USD500, and use equally as method. Then the amount would be allocated to each of the quantity of the Item, resulting your unit cost for that batch of item will be USD301, USD1 for each.
Comments
What usually you should do, assuming that you have an Invoice of Freight Cost of USD500
And previously you have a Purchase Receipt of an item receiving 500 quantity at USD 300 each
When you selected Charge Item in line item, select "Line" > "Item Charge Assignment",
Go to "Actions" tab, Get the related lines that you wish to allocate this USD500, and use equally as method. Then the amount would be allocated to each of the quantity of the Item, resulting your unit cost for that batch of item will be USD301, USD1 for each.