Last time I checked, it is a law that if you receive items in the mail unasked for, and you are legally able to accept them, (i.e., someone ships you a select fire weapon instead of the FFL, no, you can't keep it), yes, you can keep it, because of the problem people had with companies shipping items to people and then trying to charge them for them. No, I can't point to the specific law, but to the best of my non-lawyer knowledge, it is there.
As for the gold bullion...while I am reasonably certain you could have kept them if they were indeed shipped to your address and your name, the guy probably could have made a legal stink over it, just to drag you into court and make you spend that money on legal fees.
I am not a lawyer, nor do I play one on TV, but that's what I was told in the past. I've been wrong before, (just ask my wife), and I'll be wrong again, (I repeat my last), so double checking current standards is always a good idea.