I have an order that was plced this morning where the client didn't input his postal code correctly so magento didn't apply the tax rule to his order. Right no his order doesn't have any tax on it, but there should be a $8 fee included. How do I go about adding that, or do I have to cancel the order and have in do it again properly?
You can Edit order (http://screencast.com/t/agpjkxUl). In fact this will cancel existing order and create a new one, but you will not have to manually re-fill all order details for the new one.
On edit page, once you have entered the right postcode, you can click Update Items and Qty's (http://screencast.com/t/99iOdFEFv) to make sure the totals of new order are correct.