You can deselect step 1 in the post sale process, edit the WBN, then resend it with the new shipping costs. If this customer is trying to pay through checkout, have him hold off until you have done this so that they get the newly updated total.
If you need to change what the buyer entered, deselect step 2 in the post sale process. Then go in and edit the response to change shipping. I hope this helps!
