The Daily Morning Astorian, on June 11, 1889, reported that Postmaster General John Wanamaker (1838-1922) sent a letter to postmasters of 100 of the largest post offices to find out "the relative importance of mails on Sunday."
Early on, the individual post offices decided for themselves if they wanted to be open on Sundays or not. The controversy started in 1809 in Washington, Pennsylvania, when the postmaster opened on Sundays so people who only came into town to attend church could get their mail.
His church disapproved of him breaking the Sabbath by working, and expelled him, starting a skirmish between the post office and the churches.
In 1810, Postmaster General Gideon Granger (1767-1822) persuaded Congress to pass the Postal Service Act which, among other things, kept post offices open, and mail moving, seven days a week. His concerns were strictly finance-centered.
The wrangling went on for years. The Sabbath and post office dilemma became essentially moot when the telegraph was invented in the 1840s, and business information could be sent and received faster by wire than by mail.
By the 1850s, most mail wasn't moving on Sundays anymore, but several post offices were still open, sorting mail, selling stamps, etc. … Hence Wanamaker's request to find out if staying open on Sundays was still a financially worthwhile endeavor. (bit.ly/SunMail)