Post offices, as a result, were also sources of controversy. In the 1820s, leaders from a variety of Protestant denominations campaigned to end Sunday delivery on religious grounds. Similar movements would arise over the course of the 19th century. And the objection wasn't just to the Sunday-ness of Sunday delivery, to the fact that mail delivery on Sunday was a violation of the Sabbath. It was also to the social-ness of Sunday delivery. The six-day-delivery campaigns, Fischer writes, were "part of the churches’ wider efforts to enforce a 'Puritan Sabbath' against the demands of Mammon and against worldly temptations like those card games." Exacerbating the problem, from the Puritanical perspective, was the rise in immigration among Catholics, "many of whom," Fischer notes, "celebrated 'Continental' Sundays which included all sorts of secular pleasures—picnics, even beer halls—after (or instead of) church."

But the many protests that periodically sprang up to challenge Sunday delivery would, inevitably, fail. There was, for one thing, a First Amendment argument to be made in favor of daily mail delivery: To prioritize Sunday over another day—many religions, if they celebrate a Sabbath, do so on Saturday—would be, implicitly, to prioritize one religion over another. In 1828, the Kentucky Senator Richard M. Johnson, chairman of the Senate Committee on the Post Office and Post Roads and a devout Baptist, declared any federal attempt to give preference to the Christian Sabbath to be unconstitutional. The line between church and state when it came to Sunday mail delivery, he argued, "cannot be too strongly drawn."

The more pressing argument for Sunday delivery, however, was economic. "The 1844 invention of the telegraph would eventually put an end to the commercial need for daily mail," Susan Jacoby notes in an essay on religion and the Constitution, "but in the 1820s and '30s, business still depended on the government to keep the mails moving seven days a week." Back then, a significant proportion of business dealings—not just correspondence, but financial transactions—was conducted through the post office. Businessmen argued that they needed Sunday posting capabilities to do their work. And they "found allies," Fischer notes, "among some evangelical ministers, particularly Baptists, and among secular laymen who saw the sabbatarian drive as a power grab by high-status, eastern churchmen."

Toward the end of the 19th century, though, another alliance would arise: Religious leaders would join with organized labor to end Sunday mail delivery. For workers, closing post offices on Sundays wasn't necessarily a matter of religion, but it was a matter of time. A Sunday-less work week was also a six-day work week. (Though, Fischer points out, "the church-labor alliance did have its limits. Protestant ministers and the union men disagreed on how the Lord’s day of 'rest' should be spent—in religious devotion or in play.")