The DEA has never given a shit about small-time stuff. That's for state and local PD. Most of the time when a dog sniffs out a little bit of weed in the mail, a first-time offender gets a nasty letter from the postal inspector in place of their package.
Controlled delivery busts take time, money, and manpower to set up, and authorities like to make their efforts seem like they're worth the trouble. Generally $50 worth of pot isn't.