I suppose you could possibly make the case that since you didn't order it, but they sent it anyway, that it was an unsolicited gift ... however, right is right and fair is fair ... if you called customer service and told them what happened and they responded to you as they did to Slim, you mightn't feel bad about keeping it ...
But in your shoes, I'd have a real problem knowing they made an honest mistake, but keeping it without such a blessing ... and even then, I'd have a real big sense of obligation to them ...
that's MY 2cts worth