If a company sends an item that wasn't ordered, do I have to return it or keep it and not have to pay?

The right thing to do would be to return it.
The legal thing to do depends on whether you signed for the item when it arrived in the mail. If you didn't sign for it, and of course never order it, then there would be no trail proving that you have it.

If fyou dont want it, you have to send it back or be charged.

If you didn't order it don't keep it send it back. This will cost you nothing.

If you keep it and don't pay for it that's wrong and most likely they will locate the item and bill you for it. Some times things slip through the cracks and people keep the item at no charge but it's a dishonest thing to do.

If you sent something to a customer and it was the wrong address would you want them to return it to you? Or are you OK as a business owner writing the cost off, purchasing another item and re-sending it out to the correct address?

Ethically, you should return it. You just need to understand that even though it's their mistake, the are human. You should let them know and return it. Or let them know and have your account be charge. Don't keep it to yourself, think about how much their inventory will be unbalanced and someone may get fired because of this.