Add Shipping Cost in the purchases "Very Important"

Add Shipping Cost in the purchases "Very Important"

Hello, In a lot of cases web have to pay antoher fee to our providers for the shipment but in some cases are included in the invoices with a line of shipment cost and other cases are not specified in the purchase invoice but we need to pay it, we need to include this cost in the purchase invoice as a cost or the item because is a real cost that will be transferred to the item in the same time that I make the purchase, In the sales invoice appears a similar option in the end of the invoice to include a shipping amount for the sales but we need it for the purchases and will be excelent if an aditional check ath the side of the amount shipiping cost space makes the amount only add this to the invoice total or distribute the amount in all de items that are included in the purchase invoice.