No. I have already "followed my conscience" and arranged to return the stuff.
I really wanted just what I asked: an answer to the question of my legal responsibility to do so. Think of it as asking for "someone to just tell" me "what I *could* have done, without legal or financial repercussion." Considering the effect of the "mistaken" order, can the delivery still be considered "unsolicited merchandise"?
It's not an insubstantial amount of "stuff." The order came to over $250. But I stress, my actions are guided by my personal ethics. The question was purely an intellectual exercise. One which I am still curious to resolve.
Since you never actually stated what you were going to do, it sounded like you were asking for advice. *shrug* You should have stated from the beginning that you were asking as a theoretical question. Slight differences in wording make all the difference in the online world - perhaps you were unaware of this? And before you point it out, yes, I'm fully aware of the tone of my posts.

Adressing the theoretical question - it's highly unlikely there's anyone on these boards who is truly aware of every legal binding surrounding this, but consider the following:
1) Which state's law applies? The goods were ordered online. Have you ever ordered something (or just looked at) that stated in the fine print "will not deliver to X state"(think about different types of Airsoft guns) or has different rules about sales tax and whatnot depending on where you're ordering from? . This seems to imply that the law of the state of residence takes priority. Also, take some instances of pornography - let's say, oh, hentai. Ever notice how when it comes to the states it gets those little warning that say "all girls depicted here are over 18"? 18's legal to be depicted in porn here, but in Japan, not the case, so even though the girls are clearly in high school they're "over 18"

Granted, that's on a national scale, but it's still the same thing: it's the law of the destination that governs it, not the law of the originator. Now, if you physically went to New York, it's a totally different issue.
2) Are you legally responsible to return it? Probably not. As I said in my first post, the customer has never been held responsible for the mistakes of the seller. I've seen many cases of customers complaining/suing/etc. a company for messing something up, but have you ever seen one the other way around?
3) $250 is, in fact, insubstatial to a company that sells many products in the $1000+ range. Going from anecdotal evidence of people who have worked in such places, including Staples, many times managers will take back expensive equipment that the customer clearly broke on their own as "defective" to avoid conflict. This implies that $250 is no great loss to them, and is somewhat equivalent to getting extra chicken nuggets with your order at McDonald's.