I don't disagree with you Tobus, but . . . (every one has a big but)

The USPS can be a special sort of bureaucratic nightmare of a workplace. It's huge and sprawling.

There may be gender differences in the dress codes for public contact employees, but, to the best of my knowledge, the dress code for plant employees is very non-gender specific. The primary concern is safety, followed by professional appropriateness (sufficiently modest, non-obscene, non-political, etc..)

If it were me, and I were inclined to fight that fight, I'd talk to a steward and feel out the possibilities. It sounds like his supervisor made the decision. Management may or may not agree with it.

The union may or may not even want to fight it. And if he grieved it and it were shot down at step two, the union may or may not be inclined to push it to step three (national level).

In my experience, management doesn't usually hold a grudge against an employee who files and loses a grievance. They often don't even hold much of a grudge against employees who win their grievances. It's par for the course. EEOs, however, are taken more seriously and, perhaps, more personally.

But, every plant is different, and it's better to pick one's battles.

This isn't a battle I'd choose.