There's a difference between an unsolicited shipment and a solicited, but erroneous, shipment
In the USPS recommended actions, there is. In federal law, there is not. The legal definition of unordered merchandise is "merchandise mailed without the prior expressed request or consent of the recipient" (See USC 39 3009(d)). The people ordered some merchandise but what they received was something else that was mailed without prior expressed request or consent of the recipient.
True, at federal law, there isn't, because contracts for sale of goods are state law under the UCC, not the federal government. Under state law in every state, it's a solicited, but erroneous, shipment, and the seller has the opportunity to recover the incorrect goods at their expense and either cure by shipping the correct product or repudiate the sale and suffer penalties for breach.
Is this true even for interstate shipments? That seems unlikely.
Of course it does. You're getting confused because you hear about interstate commerce being regulated by the Federal government, and think that anything that travels interstate must solely be under Federal law... It's not true - there is, for example, no such thing as federal contract law. All contracts are governed under state law, including contracts that relate to interstate shipments. State law applies here.
In addition, even for intrastate I think there's a good argument that such a state law contradicts federal law, if the shipment was sent via the postal service (though most such shipments are sent via UPS or FedEx, which may be different).
Not at all. As many people in this thread have said, the federal law deals with unsolicited purchases. This is not an unsolicited purchase, it's a solicited purchase where the wrong goods got shipped.
Come on, think logically - the Federal government is going to write a law that screws large corporations when some warehouse monkey ships the wrong thing? The same Federal government that, whenever copyright or SOPA or PIPA or anything else is mentioned, we claim is bought and sold by large corporations?
Also, do you have a citation (in any state) for the law you're claiming obligates the recipient (or buyer, whatever) to return the incorrect goods?
Yes, it's in the UCC article 2-508, which has been implemented in some form in every state's laws.
I spent some time looking through Utah State Code, and it appears to me that upon receipt the buyer is fully justified in accepting the erroneous shipment and calling the transaction complete, if you choose to look at it as an error in the transaction, or in accepting the erroneous shipment as unrequested goods, and demanding that the original transaction be completed.
Nope, 2-508 is also in the Utah code. And if the buyer attempted to accept the shipment and demand that the original transaction be completed, then the buyer is admitting that goods are solicited, and would be liable for conversion.
Again, logically, come on... Mistakes happen in warehouses all the time. You really think that the government has written in statutes that explicitly screw corporations and give windfalls to individuals because of mistakes?