i could understand why pills like molly or xanax got caught if shipped through customs as that might be x-rayed. how do pills get caught domestically if you're buying self-use amount? i know dogs can potentially sniff it, but i highly doubt the usps uses dogs or x-ray on first-class mail.
Profiled packages, accidentally opened packages, poor stealth, and buyers who are already suspected and being watched. Few domestic sort facilities use dogs, so that's doubtful.
It's really just bad luck or a fluke for the most part. I am aware of two cases, both very small amounts. One was ten hydrocodone, one was three oxycontin. So the notion that buying personal amounts of pills is entirely safe, while not ridiculous, is not a sure thing.