Rural Carrier Allowed to Wear Uniform?

Hey everybody! My partner works as a rural carrier and her supervisor is getting mad at her for wearing the uniform. The post master is on vacation right now so it's hard for her to actually push back, aside from filing a grievance (which seems to have gone nowhere?). The uniform helps her feel safe up in the hills doing deliveries in her PV, and she really takes pride in her work and likes feeling professional and repping the service. Plus i think she looks so cute! Is she in the wrong here? Has anyone else experienced this before? Frankly i think wanting to feel safe and look professional is a good a reason as any - especially since she bought the uniform with her own money. Thanks!