That's right, and supports what I had written earlier. '
And you have only cherry picked what you wanted to look at and ignored the rest.
From the FTC FAQ you must have missed, and within their section on unsolicited merchandise:
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.
Clearly that says it is not illegal to keep. Merely that you should notify the seller and they should make arrangements to pick up at their expense, or they forfeit their right to do so.
Keep in mind, you, when you purchased supplied money for a product. Not two products, not extra products, not anything more than what you agreed to. A vendor sending you extra product then demanding payment falls under the Unsolicited goods framework. That stops vendors from sending everyone three of what they order subsequently padding sales.
Is it morally reprehensible to keep it? IMHO yes. But in the good old USA it's not illegal.
Been here done this and won the chargeback with Visa based on this alone.