The parent you were responding to stated that people are not really rewarded for good behaviour after childhood. I'm not saying your deed was not a good one. I am saying that you are monetarily worse off for having gone back to pay and that feeling of having done the right thing, or the discount off the original price is hardly an example of good behaviour being rewarded, especially when you compare the reward for not doing the right thing. If there is only right and wrong and the reward for doing the wrong thing is greater than the reward for doing the right thing, then there really is no reward for doing the right thing.
This is why I never have conversations with philosophy majors at parties. You are crawling through that same morass of specific straw man arguments that drive me nuts.
