Nice work, Mikec, standing up for yourself.
Laws are laws for a reason.
About the extra $3.40... I would consider it payment for the time you spent educating them on the law they didn't obey and should have already known about.
Your time is worth at least $3.40 for five minutes, isn't it? And that's not even considering hazard pay for the high-stress environment in which you were double teamed by the opposition, etc.
There's a store near me that pays
double your money back if you're not completely satisfied (e.g. if that shrimp situation happened to me at this store, I would have gotten something like $19 or $26 back- depending on which price they'd decided on).
Considering that, you got a rather rotten deal.