From anything I read the method of transmittal may vary. I’ve seen FedEx mentioned as well as USPS registered or certified mail. Keep in mind, and I’m quoting from a congressional site “A Letter of Assurance (LOA) is a letter sent to the candidate from the Admissions Office by the Academy advising that the Academy is offering a Conditional Appointment to the Academy provided the candidate successfully completes the application process, the medical review by the Department of Defense, and the physical fitness assessment and receives a nomination from their Congressman or Senators. Letters of Assurance are conditional and subject to review throughout the entire application process.
Candidates receive Letters of Assurance based upon outstanding ACT/SAT scores early in the process, and other information the candidate has submitted to the Admissions Office. It is highly recommended candidates prepare well for ACT/SAT tests and take them “early and often.”