A USPS letter carrier doesn't get a separate USPS retirement and a US government retirement if he should decide to get some other federal government job. He simply gets service time ~ 20 years for just one retirement from the federal government.
What Perri Caliente is getting is a retirement for each separate job working for the same government.
With the average federal government employee starting work with the federal government at the age of 35, if he or she retires (on a "full" ~ cough, cough) retirement at age 55, the retirement will be computed at 20% of high three. He he retires at 65, the retirement will be computed at 30% of high three. If he retires at 75, the retirement will be computed at 40% of high three.
That means the just recently hired federal employee will need to make substantial savings out of his current income in order to have a plush retirement!
FYI I do not respond to posts that use terms like Perri Caliente.