Certain types of mail do run on Sunday. Priority express and Amazon packages run Sundays. Regular mail(letters) do not. By the terms of our contract if we worked 7 days in a row we would get double pay per hour on the 7th day plus a Sunday premium. It would cost way too much money for the postal service.

Delivering an extra day a week would cost millions of dollars

Soon won't deliver Saturdays, either

Sunday is considered a day of rest.