It's theft and there's literally no way you could get away with it... you could deny that it ever arrived at your house but you could never wear it OR sell it. I guess you could hide it away and leave it to your children in a will, by which point no one would be looking for it...
I can't imagine anyone sending something so valuable through the post anyway
. It would be at least a courier... and more likely to be hand delivered by the seller.