Legal Question in Criminal Law in California

Wearing the shirt is legal as long as you don't actually impersonate a postal worker -- as, for example, by accepting people's mail, trying to gain access to buildings by claiming you're picking up the mail, etc. Merely wearing the shirt would not amount to impersonation.

Even in the situations I described, I'm not sure whether impersonating a postal worker would be a crime. Wearing the uniform would be part of the deception involved in a different crime. I would need to do some research to find out whether the impersonation would be a separate crime by itself.