I've had any number of opportunities to cheat merchants, large and small, one way or another. I'm no saint, but I generally manage to resist the temptation.

This is no different. The OP made a mistake, didn't want to suffer the consequences, and lied to get out of the situation. The lying may have been creative, and the result may have been the same as would have happened the first time absent the mistake, but that doesn't change what went on.