Stop it right now! The OP did everything he could to get it back to them. They obviously don't care enough to dispatch a box for him. Apple is not the victim here, and he is not a criminal. Don't suggest such things.
I really don't care what the OP decides to do, but there's no way (at least so far...) that you can say he did "everything he could" to return something that was a mistake, and isn't really his. He did the right thing, and he did call them. But, Apple's a big company and it's easy for it to slip through the cracks in any big organization, depending on who happens to be on the other end of the line. We all know this from our various customer service experiences. If it was a lost dog, I don't think it would end with one phone call if the word didn't get through to someone who actually understands the situation. Most of us would probably keep trying to contact/locate the owner in that case, because we would know it was someone else's pet (property.) In this case with a brand new MBP, it's easy to see how tempting it is to just sit on it now, since a single phone call was made, hoping nothing happens, and the "loser" would be a big company.
The OP did say he would call them again, which at least shows he's making an effort. Maybe he might get through to someone who is more able to put the pieces together, and resolve this. I guess we'll have to see what happens there. But until someone on the other end actually realizes what happened, you can't really say they "don't care." I'm sure someone in management might take more of an interest in the matter, even if a clerk doesn't.
In the end, it really is a matter of character, and everyone has varying degrees of integrity that we all can live with down inside. Only each of us knows what we'd do in the same situation, or maybe we don't know until we face it. Who cares what other people would do? The only question that matters for any of us is, "what would I do?" Then just do it, whatever it is, and say "hello" to your own true self.
It's none of our business, really, in the end. Good luck to the OP, whatever you decide. Maybe Apple will be happy to let you keep the MBP, but somehow I doubt that would be their practical business decision. If they do, I'd actually be happy for you. It would be a decent gesture on their part, but if it's a mistake, that's not the same.
The ball's really in your court.