That is not the law, it is only a suggestion. It might be the "right" thing, but you are certainly not legally required to do this. These laws were put into place to protect consumers, not corporations. It provides incentive for the companies to play by the rules. When they don't, or have poor business practices, these are some of the consequences. Corporations have enough power to steam roll the little guy as it is. These rules are in place because their mistakes can have costs or negative impacts on you, the consumer. The OP now has to waste his time dealing with this issue that was not caused by him, and that he didn't ask for. The OP should definitely know his rights, before the company tries to charge or intimidate him for their mistake.
I'm not going to impose my morals or judgement, just want to educate the OP as to his rights so he can decide what he wants to do.
I will say that I have had a very similar situation, in that I received something from Amazon that I didn't order. I googled the original person who placed the order and emailed him about it. Sent him his item (at my own expense), and got a nice thank you in return. I wasn't required to do this, but felt very good about. I could have kept it, or returned it to Amazon (with some amount of hassle, I'm sure) to let them deal with it. I was most comfortable with my choice to deal with the guy directly.
P.S. Karma does not exist.