I disagree totally! If the OP made the code up to just try it on a goof and it worked, then she is stealing. However, if the code is published in various places around the Internet, then the code is publically available, just like many other publically available discount codes for Avis, Hertz, Hilton, Marriott, and many other travel companies.

If Avis did not wish for the code to be used, then all Avis needed to do was change the code, often, and make the changed code known only to the parties which Avis wishes to use that code.

Obviously, Avis has a computer system, and they can track the usage of their own discount codes, and if they cannot do that, then that is Avis's problem, not Avis's customer's problem, is it? If it was a problem in Avis's eyes, then Avis could darn well do something about it, yet they have chosen not to, so what's the beef?