Kevin Drum, who is maybe my favorite online pundit, thinks Trump's antipathy towards the postal service can wholly be explained as a revenge tactic against Jeff Bezos and Amazon. And at first, that's easy to believe because Trump does often seem motivated by irrational desires for revenge against anyone who doesn't smooch his tuchis.
But Trump is hardly the first prominent Republican who's had it in for the post office. So I tend to think there's more to it than that.