I wonder if there are age restrictions for Luxury at National. If I recall correctly, in another thread, you said you were under 25. I also remember a few years ago, underage renters said they had no issues reserving and renting luxury cars at Hertz with their Underage Coupon or Gold Family. Policies might be different, so you might want to call to avoid surprises.

Also, are you using the luxury free day for an expensive one-way?