Answer:
"Even in these times where privatization of governmental service is a popular political concept, it would be very difficult for the federal government to entirely privatize the U.S. Postal Service because the delivery of mail by the government is provided for in the Constitution and under the Private Express Statutes" according to John J. Zodrow, an authority on Postal employment and labor relations. "It would literally take an Act of Congress or a Constitutional Amendment to alter these provisions."
According to the USPS at its website, the Private Express Statutes are a group of laws under which the U.S. Postal Service (USPS) has the exclusive right, with certain limited exceptions to carry letters for compensation. The Statutes are based on the provision in the U.S. Constitution that empowers Congress "to establish Post Offices." Congress enacted the Statutes to protect the USPS and thereby enable it to fulfill its mission of providing mail service to all parts of the country at uniform rates (even in remote areas that otherwise would see not see affordable service by a private company). The Statutes enable the USPS to fulfill its responsibilities by preventing private courier services from competing selectively with the USPS on its most profitable routes, to the exclusion of expensive or not profitable areas, such as rural areas.
For the purpose of the Private Express Statutes, a letter is defined as a message directed to a specific person or address and recorded on a tangible object. A more complete definition can be found at title 39, Code of Federal Regulations (CFR), 310.1. The Statutes provide that letters may be transported outside the U.S. Mail system only if one or more of the exceptions apply, or appropriate postage is paid. The basic prohibition is against private carriage of letters for other persons without payment of postage.
The law allows for the private carriage of letters under certain circumstances, including:
Letters for which sender has paid at least 6 times the price currently charged for the first ounce of a single-piece First-Class Mail letter.
Letters weighing at least 12-1/2 ounces.
Letters sent with and relating in all substantial respects to the cargo that they accompany.
Letters carried by the sender, recipient, or the regular, salaried employees (not independent contractors) of either party.
Letters carried without any financial or other valuable compensation.
25 or fewer letters carried by special messenger on an infrequent, irregular basis for the sender or addressee.
Letters carried prior or subsequent to mailing.
Properly marked letters whose value or usefulness would be lost or greatly diminished if Letters not delivered within certain time standards listed in postal regulations ("extremely urgent" letters).
Letters addressed to campus destinations that colleges and universities carry in their internal mail systems for their bona fide student or faculty organizations.
Letters carried from a point in the United States to a foreign country for deposit in the foreign country's domestic or international mails for delivery to an ultimate destination outside the United States.
Data processing materials sent between an office and a data processing center, within certain delivery time standards listed in postal regulations.
Certain international ocean carrier-related documents accompanying cargo.
Advertisements enclosed with merchandise in parcels or accompanying periodicals under certain conditions.
"Accordingly, some non-delivery services formerly performed in the past exclusively by the USPS or it's predecessor -- the Post Office Department -- are now performed privately. One example is the pre-sorting of bulk rate mail for a discount before it enters the Postal system. The transportation of mail also is performed largely by private carrier. As is the delivery of overnight and second day express packages by companies like Federal Express and the delivery of parcels by United Parcel Service. But the delivery of personal letter mail, and the operations directly necessary for the delivery of such mail, seems to be well-protected against privatization. This appears to include the collection and sorting of mail for delivery. Postal public sector employees unions are not going to concede their rights to this work, especially given such strong legal authority," according to John Zodrow, who authored the textbook Postal Service Labor Arbitrations.
More information regarding exceptions and suspensions to the Private Express Statutes is available in the federal regulations at 39 C.F.R. ยงยง 310.3 and 320.