Jevons Brown, 58, had pleaded guilty in August to a misdemeanor charge of mailing injurious articles.
Brown, a veteran, had become frustrated with a lack of employment opportunities and sent the feces to employees of various companies.
Twenty packages were traced to Brown, U.S. Postal Inspection Service spokesman Dan Taylor said.
Brown apologized in court on Friday during the sentencing, the St. Louis Post-Dispatch said.
"I'm sorry. This will never happen again."