Most coupons, upgrade deals etc. are only good if you return the car to the renting location. Sometimes in the spring you get some deals with one way returns on cars that the companies want to move North after snowbird season, but your timing is wrong on that one.
Also, make sure you know what the dropoff charges are. My experience has been that the rental car websites do a lousy job of telling you what they are so I would give the agency a call to verifiy anything. I know Hertz usually charges mileage for example and I think National does to. However, that does not always show on the booking screens.
