Make sure 'Allow edit of closed purchase orders' is selected in: File | Configuration | Options Tab | Purchase Order Options.
Then find the closed purchase order and add a fake item to it and save it. This will place the purchase order in partial status (find it in the Filter drop down near the top the Purchase Orders window).
Find and select the partially received purchase order and click the receive button.
Type the additional shipping charges to the Current Shipping field and click the Commit button, then click Yes when asked to modify the quantities.
On the next dialog box select the top option 'Close the Purchase Order' and then click 'OK'.
Done.