Hi Parth,
as told by Mani Kumar, pls follow the same process.
If your freight vendor is not fix while creating po, then..pls dont worry...you can enter the freight amount in PO & let GR post along with freight value.
As per sap standard, you can change "freight vendor" (plan delivery cost vendor) even after GR post.
Vendor will come into picture while MIRO.
You can post MIRO for plan delivery cost to your desired vendor..you can invoice vendor code in MIRO.
Hope, you got it & this will resolve your problem.
Regds,
Nilesh