1) "For your theory to hold up it would have to hold up across the board." This is not true at all. LE uses different means to gather evidence, and they use different means to corroborate evidence. If LE traces an IP address that was used to purchase N to a specific household, first, there is no guarantee that someone in the house is the person who ordered the N. Ever hear of war-driving? Anyone could hack into your wifi and use the connection for nefarious means. Moreover, depending on how LE got their information determines how much they may know. If they intercepted the N at customs, and let the shipment go through, they may have "a" name and address on the parcel, but it may not correspond to the person who owns or rents the house to which it is addressed. The person to whom the parcel is addressed may not even be known to live in the house to which the parcel is addressed. If that's the case, LE would have to make more vague inquiries if they show up at the door. And as mentioned before, the person who ordered the N may have just piggybacked on your wifi. Happens all the time. Another thing that happens frequently are strangers that use addresses that are not their own. One case in point is my mother's house. She passed-away last year and the house went up for sale. Pictures were posted online, which showed the entire house, inside and outside. It was obvious that the house was empty, because all all of the interior pictures showed empty rooms. Soon after the house pictures were posted online, mail address to a person I did not know started arriving in her mailbox. It was from the local department of Job & Family Services, which handles welfare and unemployment benefits. I returned the mail, but received more a few couple weeks later. When I called Job & Family Services it was explained to me that mailing addresses of empty houses are used frequently to fraudulently apply for unemployment benefits. The person behind the fraud tries to retrieve the mail before someone else gets it, because much of the correspondence requires some type of action. In this case, they were not successful because I went to check the house every single day. But, it happens a lot. Same thing could happen if someone orders N and uses the neighbor's address. They would just have to beat the neighbor in collecting the package, which can oftentimes work, since the neighbors often work during the day and no one is home.
LE can get info from seizing packages at customs. They can get info by tracing the Internet service that was used to order. The may even be getting info by tracing the movement of the cryptocurrency used, which may vary some, in how they go about it, at least, between the different cryptocurrencies. What information they do get determines how they go about their investigation. And if you don't think they can do all that, I don't know what to tell you. If they corroborate a name used for the payment, to a same name at a house, and the Internet service account, also, matches someone at the house, then I would imagine that when they come to the door they would ask for a specific person. If a trace of the cryptocurrencies does NOT match the name of the person who has the Internet service at a particular house, or doesn't match the owner of the house, or person renting the house, where the order originated, then LE would be a little more vague and inquisitive when they come to the door.
Nothing has to hold up "across the board", as you put it, because of the number of different avenues available for investigation, and the varied information that might be gleaned depending on what is found out. You seem to want to argue for sake of arguing. You seem to be stuck in some kind of "tunnel vision" and are unable to see a forest through the trees. LE investigations are not some linear equation, nor are they mathematical. My intent was to get people thinking about the unknown unknowns. You have no idea of all the tools available to the government. That's a fact. You're kidding yourself if you think the only way LE can get information about N crossing the border is at customs. And if you think that using encrypted Proton mail, and encrypted cryptocurrency transactions, and all of that is not discoverable by LE and the government, then you're deluding yourself. Hell, the US probably is getting lost of help from the Mexican authorities. It wouldn't take much to secure their help. The Mexican government is always in need of something from the US. Tit for Tat, the US needs something, too. And it's well know how hardcore the Mexican government is. They don't have the rules like we do here.
Right now, it seems, the US, and a lot of the Western world, is hellbent, at least, on making a disruption into this aspect of suicide. LE may not be able to be on it 100% of the time because of other investigations they must do. But, they seem to be sending some type of message, and it seems that it's a somewhat effective one, at least, at the moment. I'm sure, overall, it's a very, VERY small percentage of suicidal persons who try and acquire N and who use it to ctb. Maybe that's why they're picking on this method, because it's the one they can stop. They can't stop stores from selling ropes. Same goes for selling charcoal and tents. And it's pretty hard to stop people from jumping from high places. This is one where they have the tools to stop it, and I bet they're using every one at their disposal.