Supplier Invoice does not link with PO

Hi Gurus,

             When we try to create New Supplier Invoice Without PO , reference can be made with the existing PO which not fully Invoiced,  In the new Invoice - Item tab, Basic Data- Subtab,  General Information- Type: Additional Cost Debit Items, it not referring to the  PO. In the document flow, PO is not linked.  Our scenario is the we want to add freight or other charges for the supplier. PO to be referred to the Supplier.

Thanks

Vijay