> I never thought of this but it would be surprising if the USPS _didn't_ maintain some sort of chain of custody of mail items, at least at the bag level or wha

> I never thought of this but it would be surprising if the USPS _didn't_ maintain some sort of chain of custody of mail items, at least at the bag level or whatever unit they use. Probably so. And in civil contract law we almost presume the USPS works more or less like a black box and always functions correctly. I mean, more specifically here, you might not have to question the USPS but might have to question the lab techs, or some company representative, and open up debates about what safeguards they use to prevent samples from getting mixed up. Lots of openings you could avoid by just testing on site, so that will always be easier. You generally just do whatever you can to remove extraneous witnesses to simplify your argument. The subtle trick with US criminal law is that it is still a "testimonial" system. You have to have someone vouch for every piece of evidence. If you take photos of the crime scene you technically have to ask someone questions like, "do these represent the scene of the crime?" ("How would you even know? What makes you an expert on what this place looks like?") Some of this stuff can get waived but in contentious cases you dot your is and cross your ts. Physical evidence is a bit like a hacked on extension of an old oral only court system. You originally just asked a bunch of villagers what happened and hoped they were sufficiently scared of swearing oaths to not lie.