FWIW packages sent to member offices get scanned by a mail processing center off site, I'm not sure if they'd actually deliver the coal on to the office or not =P.
The dust of coal itself can also have adverse health affects if inhaled. You might want to properly shrink wrap the coal. You don't want some poor unpaid intern to get a whiff of coal in their lungs between when they open the package and throw it in the trash. The workers who inspect all packages might not appreciate it as well.