Ohhh okay so general public members *do* need a license to purchase. "Ignorance" of the regulation could result in a fine, potentially, but it isn't a crime. So they could totally confiscate it at customs and hold it pending you providing proof of a license to possess it, but I'm not sure what search and seizure statutes would apply after its already in your hands (the only laws I found were concerning illegal substances, not regulated ones.) It sounds like they have enough legal leeway to fuck with you about it, if they choose...they always like to leave themselves that wiggle room on the off chance someone they bend over *does* bring them to court over it. But it sounds like it already got through customs, so you should be straight. If it were me, I'd hide it well as soon as I get it...if the odds come up that anybody comes asking, I'd say I disposed of it when I got it because I meant to order something else (anything totally legal will do as an excuse.) You can play the whole "I don't have to say shit to you" routine, but in a case like this its probably better to have a well-rehearsed, believable excuse to get them off your back. Playing the right to remain silent card will just make them more interested.
...but you'll probably get your SN in a couple of days and never hear from a soul about it, lol. I'm just a twitch when it comes to stuff like this. Nobody gives a shit about SN in the US, and I've still already written an entire song-and-dance to do for the law just in case