Wrong.

Avis' practice is far from misleading. You signed a contract with Avis for $69 for a 1-day rental.

You then called to amend the contract and you were quoted a new rate for the new contract. You either accept the contract amendment or you do not. Your choice.

As to simply not returning the vehicle and paying the late return fee --- that is usually fine unless you get a nervous nelly clerk who reports it as stolen and then you have some serious issues and explaining as to why you did not call in.

All about risk management and risk tolerance and choices.