Lysander Spooner's Post Office:
In an address given in 1844, abolitionist Charles Dexter Cleveland contended that the U.S. Post Office not only suppressed abolitionist writings, but also provided a massive subsidy from the profitable Northern routes to the unprofitable Southern routes. It occurred to me that this consideration could well have explained why abolitionist Lysander Spooner decided to establish his private American Letter Mail Company in 1844 to compete with the Post Office. While obtaining additional information about Spooner's enterprise, I came across this recent account of Spooner's venture: Lysander Spooner and the United States Postal Monopoly by Michael Billy.
On May 11 the US Post Office is raising the price of stamps by 2¢. Even in the face of increasing prices, many people will argue that the Post Office is necessary because a private organization could never perform these functions for a similar cost. The story of Lysander Spooner, however, might rekindle the debate over the necessity of a monopoly Post Office.
Although Billy does not pursue the antislavery angle, he does supply a number of interesting details. For example,
Spooner ran a front page ad in the New York Daily Tribune announcing the creation of the company, while stating that his rates would be 6.25 cents per half-ounce letter, or 20 stamps for a dollar. He also stated that delivery would be daily, or twice daily between New York and Philadelphia. The most audacious part of the ad, however, was his direct challenge to the Constitutionality of the Post Office: "The Company design also (if sustained by the public) is to thoroughly agitate the questions, and test the Constitutional right of the competition in the business of carrying letters - the ground on which they assert this right are published and for sale at the post offices in pamphlet form." Spooner wanted competition to be legal.

He even went as far as to send a personal letter to the Postmaster General informing him of his intent to form a letter delivery company. In the letter Spooner said that he proposed, “soon to establish a letter mail [company] from Boston to Baltimore. I shall myself remain in this city, where I shall be ready at any time to answer to any suit. . . .” Accompanying the letter was a copy of his pamphlet, The Unconstitutionality of the Laws of Congress Prohibiting Private Mails.
If you have any interest in Spooner (or in alternatives to the Post Office), you should read the whole thing. Spooner's essay against the constitutionality of the postal monopoly can be found here.