And I just complimented MCO in another thread....

First and foremost any Avis location staff has proved to be worthless if it is after hours presumably because most managers and office staff are gone. When you picked up the car what did your contract say on it? Do you have a screen shot of the original booking? Did you ask for an upgrade to which point they did but charged you for a higher priced vehicle in the process?

I had a similar issue with Hertz a while back and customer service got me nowhere until coincidentaly just after I filed a credit card dispute.