Didn't realize that. I thought their jobs were styled after most government positions, Civil Service job titles that bring higher grades, and more salary, and performance evaluation raises once a year. In the job I worked, every position from Lieutenant down is based on Civil Service exams. Captain and up are political appointees as I'm sure the USPS has that too. I know the Postal Service is Unionized, which caused me to believe that their hiring for permanent employees required the passing of a Civil Service test.
USPS hasn’t been Civil Service since the early 80’s...and the only political appointee is the Postmaster General of the US.