First class letters are exclusive to Usps Safety angle. Every item going on domestic aircraft is scanned for bombs etc.. Safety is a Usps priority. After Unabo

First class letters are exclusive to Usps Safety angle. Every item going on domestic aircraft is scanned for bombs etc.. Safety is a Usps priority. After Unabomber where all mail from west coast went via truck till he was caught and 911 Usps is insane about nothing blowing up an aircraft with postal stuff in its manifest.