How to add freight charges on Existing Requisition or PO?

Summary:

Dear Experts,

We are continuously facing an issue to inward material on time due to the below reason 

The Freight for a particular courier is calculated on when its dispatched , hence its not possible for the supplier to know the freight before hand

Now , when we have already released the Po for the material it becomes increasingly painstaking to get the PR and add the value into the Po and send it for reapproval

During this entire process the following issues/Demerits are Observed 
1. The Payment of the supplier is delayed 
2. The GRN and Quality process are delayed thus leading to Delay in further Shipping of the Product