The series comes from the 'Current Employment Statistics (Establishment Survey)'
The source code is: CEU9091912001
Source: US. Bureau of Labor Statistics
Release: Employment Situation
US. Bureau of Labor Statistics, All Employees: Government: U.S. Postal Service [CEU9091912001], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/CEU9091912001/, July 1, 2015.