....Last time I checked, when you send mail you aren't required to use [the Postal Service]..."
Actually, you are:
"On March 3, 1845, Congress passed a broad statute which set the parameters on the postal monopoly for years to come. The statute prohibited establishing a private express for the carriage of letters; prohibited sending a letter, carrying a letter and transporting a letter by private post, and penalized the sender, as well as the person who transported the letter."
FEDEX, UPS, and DHL are able to get around these restrictions because they offer specialized services (next day delivery, parcel shipment).
Read more about it in Carl Watner's excellent overview of the history of private competition with the U.S. mail, "Plunderers of the Public Revenue": Voluntaryism and the Mails, The Voluntaryist, Number 76, October 1995.