It sounds like Nintendo should ask the courier company to fix things, not you. In a way, he is also a victim of the courier company's bad service, except that the mistake was in his favor.
And also, think about what that implies. How far are you supposed to go if they screw up? What if they do it again, with something more valuable?
You can't apply human morality to large corporations. If someone on Etsy or Ebay made a mistake like that, then yes of course you should ship it back. But corporations of that size are just systems. They will not be hurt or offended if you don't return it, and they won't be gracious if you do.