ribit,
Continuing with your assertion that the USPS has competitors...
the Post Office has competitors to SOME of its services. But not all. And not to its main service - 1st Class Letter Delivery.
From Wikipedia,
Article I, section 8, Clause 7 of the United States Constitution grants Congress the power to establish post offices and post roads, which has been interpreted as a de facto Congressional monopoly over the delivery of mail. Accordingly, no other system for delivering mail – public or private – can be established, absent Congress's consent. Congress has delegated to the Postal Service the power to decide whether others may compete with it, and the Postal Service has allowed an exception to its monopoly for extremely urgent letters.
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