Hi Rama ,
as Dev Said you can use the unplanned delivery cost for this scenario , if the delivery cost is not fixed
second , if it is fixed as per total PO quantity (like 2500 for 20 ton) then maintain in PO and then while doing freight invoice go for partial invoice.
Third way will be subsequent debit , which can be issued to the freight vendor.
forth way will be from direct finance entry.
search on net you will get some more ideas.
Thanks
Kunal