Doesn't congress make a law saying breaking x regulation carries y penalty and at the same time gives the agency power to decide what constitutes breaking x? I

Doesn't congress make a law saying breaking x regulation carries y penalty and at the same time gives the agency power to decide what constitutes breaking x? I know for a fact that BATFE does this rather frequently. Also, thanks for the clarification about the secrecy of US mail. I apreciate it