The free market can be defended with a variety of plausible philosophical arguments. But most people care more about what "works" than what is "right." So in the constant struggle between free markets and the government, it may be useful to maintain the government’s monopoly in delivering first class mail. That way when someone suggests a new intervention by the government, the free marketer can refute them with two persuasive words: "post office."
When it comes to first-class mail, the U.S. still does things the old-fashioned way, with one Postal Service. Not so in places like New Zealand and Sweden, which have opened their mail systems to private companies. The latest is Britain, where the Royal Mail lost its 350-year monopoly on delivery. At least 14 companies are now competing to sort and transport mail. British regulators believe competition will be good for the mail system. Japan is soon to follow. With the recent rise in U.S. stamp prices, expect more calls for privatization here too.