In the case of Air Courier Conference of America v. American Postal Workers Union, AFL-CIO et al., 1990, the U.S. Supreme Court ruled in favor of private express mail companies and against postal workers who argued that these companies were infringing on the United States Postal Service's (USPS) monopoly over letter delivery as established by Congress under the Private Express Statutes (PES). The court held that USPS had statutory authority to suspend its monopoly over certain types of mail services if it determined this would be beneficial for public interest. In this instance, USPS had suspended its monopoly allowing private courier services to deliver international letters which was challenged by American Postal Workers Union. However, their challenge was dismissed with a unanimous decision from the Supreme Court stating that such suspensions are within USPS’s power and do not violate PES.

In the dissenting opinion for AIR COURIER CONFERENCE OF AMERICA v. AMERICAN POSTAL WORKERS UNION, AFL-CIO, et al., it was argued that the majority's interpretation of the Private Express Statutes (PES) and its exceptions were too narrow and contrary to Congressional intent. The dissent believed that Congress intended a broad monopoly for the Postal Service over letter mail delivery but also provided exceptions allowing private carriers to operate in certain circumstances. They disagreed with the majority’s view that these exceptions should be interpreted narrowly as only applying when there is an urgent need for swift transportation of letters beyond what USPS can provide. Instead, they contended that these exemptions should apply more broadly to any situation where private carriage would not harm USPS’s ability to perform its public service duties or undermine its financial stability. Furthermore, they criticized how this decision could potentially stifle competition and innovation in mail delivery services by limiting opportunities for private companies.