Deceptive behavior is never excused by legal agreement.

The agreement can easily be invalidated if the rep's actions can be shown to be intentional. This is even true when the contract has language that indicates it supersedes any verbal agreement. In some states this could rise to the level of being a criminal matter as well (fraud/forgery statutes sometimes include when a party changes a contract after agreement without disclosing the changes).

That said, IMO this is a customer service issue, not a contractual one. And the number of annual rentals isn't relevant, nor is the actual text of the agreement. The agent represented one thing, then enacted another. The issue at heart isn't the law or the contract, it's saving customer goodwill.

From a purely data mining standpoint, one complaint / occurrence of this type would likely generate nothing, but over time unethical people tend to act, well.... unethically. That would hopefully generate complaints over time and that pattern would cause action. Avis as a firm, and its licensees, have a vested interest in acting appropriately, and deception such as this would be an aberration rather than the norm.

Best of luck on the resolution.