"Is there a different law I'm unfamiliar with that you're referring to?"
The other law I mentioned DOES apply. No, I don't have a citation at hand, but I sure hope you see that this one simply isn't relevant to the case described in TFA
I don't see that at all. USC 39 3009(d) seems perfectly clear to me, and to cover this situation quite well. If there is another statute that is a better fit then that one would take precedence, but in the absence of the law you claim exists but cannot cite, USC 39 3009 would apply just fine.
Further, if the law you reference exists, I can't find it. Clearly it would belong in Title 39 - POSTAL SERVICE. Within Title 39, the obvious place for it is part IV - MAIL MATTER. Within part IV, chapter 30 - NONMAILABLE MATTER doesn't seem like an obvious fit, but it's better than any of the others. And finally, within chapter 30, there's nothing that fits other than section 3009.
Perhaps the law you're referring to is a state law, not federal? If so, it may or may not be valid, to the extent it conflicts with federal law. But even to the extent it is valid, it applies only in the relevant state, not the US as a whole.