I think that the way the Avis local clerk dealt with you was tacky and wrong all around.

However, all that being said, many Avis non-airport locations are odd ducks. Some of them are franchisees, and others are an odd combination of corporate owned, local partner operated. What local partner operated means is that the cars are owned by Avis corporate, but the business is managed by a local station owner in exchange for a salary plus a portion of the profits of that location. This is different from franchisee locations, where both the business and the cars are owned by the franchisee, and all of the profits from that location, less a franchise corporate percentage, belongs to the franchisee.

In NO CASE should you have been lied to, even if that location does not carry premium or luxury cars, and even if the coupon you had promised you an upgrade (the question here is, did the coupon limit upgrades to full size max). I have been to a few locations which do not carry anything higher than full size, and the most recent visit to one of those locations, I did get a brand new 2010 Taurus, but I knew what I was getting in advance, and did not complain. That does not change the fact that you were lied to at the very least, and denied what you were entitled to at the very worst.