Anyone know how to add shipping cost to a purchase order? I dont even see the option on custom transaction form for Purchase order to enable a shipping cost. or Does it need to be a line item?

It would either be added as a line item, under the expenses tab, or as a landed cost on receipt/billing.

I agree with Simon, shipping costs need to be part of the business process, if they would be a regular thing. Read about landed cost functionality that allows to add estimated costs and can be corrected to actuals at the time of billing. If this is a one off thing, then just create a non inventory item for shipping charges (if it is not already present) and then add it to the PO as a separate line.