The selecting officers for postal hires have a choice of the top three candidates for a position. Disabled vets go to the top of the list. Five point veterans with wartime era service, like Desert Storm, are mixed thruout the list with other candidates according to test score order. The selection must be made among the top three candidates on the list. That said, the selecting officer feels obligated to select a racially balanced and gender balanced workforce. Justification must be provided if this goal is not achieved, bonuses may not be awarded by higher ups.
Your concerns in that regard would seem more in tune with such as agencies as National Forest Service, National Park Service, etc.
USPS, for the last 30+ years, has rarely hired "outside" for higher level positions.