Where does it say that in law? Consumers make law up to suit them
It is considered "Unsolicited merchandise" under state and federal law.
From the FTC:
"What do you do when you receive merchandise that you didn?t order? Under the Federal Trade Commission, you don?t have to pay for it. Federal laws prohibit mailing unordered merchandise to consumers and then demanding payment.
Here are some questions and answers about dealing with unordered merchandise.
Q. Am I obligated to return or pay for merchandise I never ordered?
A. No. If you receive merchandise that you didn?t order, you have a legal right to keep it as a free gift.
Q. Must I notify the seller if I keep unordered merchandise without paying for it?
A. You have no legal obligation to notify the seller. However, it is a good idea to write a letter to the company stating that you didn?t order the item and, therefore, you have a legal right to keep it for free. This may discourage the seller from sending you bills or dunning notices, or it may help clear up an honest error. Send your letter by certified mail. Keep the return receipt and a copy of the letter for your records. You may need it later.
Q. What should I do if the unordered merchandise I received was the result of an honest shipping error?
A. Write the seller and offer to return the merchandise, provided the seller pays for postage and handling. Give the seller a specific and reasonable amount of time (say 30 days) to pick up the merchandise or arrange to have it returned at no expense to you. Tell the seller that you reserve the right to keep the merchandise or dispose of it after the specified time has passed.
Q. Is there any merchandise that may be sent legally without my consent?
A. Yes. You may receive samples that are clearly marked free, and merchandise from charitable organizations asking for contributions. You may keep such shipments as free gifts.
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Actually, those laws refer to unsolicited items, to prevent people/companies/etc from sending random items to people and then billing them for it.
In this case, it wasn't unsolicited. It was a shipping mistake involving a product (or products) that had beed requested. There is a legal obligation to attempt to return the extra merchandise. Much like if you suddenly find $1m in your bank account...you can't keep it.
See above from the FTC