You can use a delivery type accrual condition , enter the transport vendor in the condition details in PO . This way you can do Invoice for plan costs for the transport vendor based on the same PO , The transportation cost will get added to material cost during GR.
You can enter an expected amount for the condition and can do the Invoice for the correct amount. Since the invoice value has precedence, this will be the value that goes to material assuming there is sufficient stock to cover the difference,
If you are raising a separate PO for transportation, then this value can be added as an accrual Freight condtion. This value will get added to the material while GR. Give transportation GL account as the accrual GL