There are several letter carriers in our family. What I've seen, through them, is that the management of each post office can vary greatly. So where one carrier might be able to take a couple minutes to go out of her way for her customer's convenience, another carrier in another office might get in trouble for not following SOP to.the.letter. KWIM? (In some places carriers are actually being watched or timed in one way or another.)
Being a letter carrier is a physically demanding job. Can you imagine 30 years of stairs for the city/suburban carrier? The good carriers realize that we are their customers and they are worth their weight in gold! The carriers with no regard for customers or basic rules of conduct (no cell phones, smoking etc) make me crazy!