Adding Freight Charges To Purchase Order?

Is there a field available to add incoming freight charges to a purchase order?

  Shall I create a Service type item in inventory pointing to a new incoming freight GL account? Or is it better to add the freight charges in the Supplier Invoice screen under the GL Items For This Invoice? I think I would rather have the freight to be listed on the purchase order for PO historical record.

  Same with the occasional paying of taxes - Sometimes it's easier to pay the tax on the one-off vendor purchases.