Say a LE wants to build a case on a vendor or in short they want to bust him. They track the packages sent to buyers. But they let them go through. For example a user orders a few gramms hash or marijuana, it gets shipped out, LE tracks the package, makes photographs or whatever, but let the drugs reach the buyer. They never get in cots the with the buyer nor do they raid his house or similar. After 1 year, or even later the buyer gets a letter that legal proceedings are made on him (hope that's written correctly, it's not my first language) because of ordering drugs from a vendor who got busted. They only say that they let the order arrive to get to the vendor. They don't tell what was ordered or how much. The buyer answers to the letter that he will not make any statement or testimony about this.
If they have no evidence (the drugs) how will they make a case against the buyer? If they have no evidence. Couldn't everyone be ordering on the name of him and steal it from his mail-order? It could be that there was nothing in the envelope, maybe the vendor sent empty package?
The best would be to just say nothing I guess. Who says nothing can say nothing wrong, right? They have no evidence the buyer really ordered and received the package. The shipping method could have been only normal without tracking but maybe it was a tracked letter, but without signature. Only a so called throw-in-tracked-letter. It only states the pack has been thrown in the mailbox, not that it was received by a person. That would need a signed tracking. Or a signed-tracking which is only handed to the person named on the envelope, and only with signature. Does someone have any clues on that?
Hypothetically, if they let package arrive, and see proof of you taking package from porch etc that would probably be enough. Package with your name, known to have drugs, picked up by you and not reported to post office enough evidence to get you. If they are just basing it because it arrived at your house, be hard for them to get you on that.