^^This^^
Let's be honest, here. This wasn't an unsolicited gift mailed to you by a donor. You placed an order for the product. While you did everything you were supposed to do to cancel that order, for whatever reason it didn't get canceled. The mistake was on the carrier's part. Should you be able to profit from that innocent mistake? Do you wanna "stick it to the man"? Only you can make that decision. I can say with full confidence, however, that you would do whatever you could to NOT allow the carrier to profit from an honest mistake YOU made, and you would (rightfully) conclude that it wouldn't be fair or equitable for the company to be able to do so. So why are you looking for internet justification to do that very thing? And just because some posters might give you that sought-after justification, does that make it right?
You're the one who has to look yourself in the mirror after you make your decision. You're also an anonymous poster, so no one - except you - will ever know the decision you ultimately make. Remember, though, that character is doing the right thing when nobody's looking.