This is confusing because you state that, "I used a corporate code with CDW included (my reservation confirmation clearly listed CDW included)" -- but then you imply that National charged you extra for it. How could they charge extra to add on something that was already included? It would help if you could provide a screenshot (with any identifying information blacked out) or copy/pase the actual line items from the rental agreement and/or final bill for reference. Something isn't adding up here...