I received something in the mail that I did not order. Do I have to pay for it or send it back?

No, neither. It is illegal to send unsolicited goods through the mail and then bill for them. If you receive something in the mail that you did not order, you may keep it. An exception is that a charitable organization may send a gift requesting a donation without violating this law. However, you are still not obligated to donate nor to send the item back if you choose not to donate.  

For more information, see: 39 U.S.C. § 3009 (2015).