The lesson is - some people will try to cheat you but they can only do it if you let them. And they're much more likely to try to cheat people that they think are easy marks. Once they see you aren't, they'll probably back down.
EXCELLENT and bears repeating! Key work is "probably". Sometimes, no matter how good you are at being properly assertive, you can't win in the first round.
While Chris may have felt good shouting to other customers while leaving, it didn't get his root needs met. This is why I defer these matters to my wife.....she is awesome.