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If it's addressed to you - you can keep it. But if not - you'll commit mail fraud.

The law does not declare misdelivered merchandise to be a "free gift" like unordered merchandise.

In fact, the code states: "Whoever, without authority, opens, or destroys any mail or package of newspapers not directed to him, shall be fined under this title or imprisoned not more than one year, or both." 18 USC 1703

Does this apply only to USPS deliveries, or are private shippers like UPS, FedEx, etc. included?

That would depend entirely upon the legal definition of mail.

So you don't have to return it, but you can't open it?

That doesn't really cover misdirected mail though just companies sending out merchandise and trying to get paid for it.