Here's an interesting article in Lapham's Quarterly about the early postal system:
“The post-office system offers a facility for clandestine correspondence which no respectable father or mother on the European side of the Atlantic would think of without a shudder, if it were proposed to give our young women a similar privilege.” —Blackwood’s Edinburgh Magazine, January 1867
Communication of and by women has always struck fear into the hearts of men (see: novels; epistolary), but until the middle of the nineteenth century it was largely manageable—husbands and fathers, even servants, monitored a lady’s letters, and the wild fluctuations in cost of mail kept all but the wealthiest of girls and women from taking pen to paper on a regular basis. That changed with the standardization of postal prices in 1845. The cost of mailing a letter was reduced to three cents, making the mail accessible to working women, middle-class housewives, and schoolgirls with pocket money. Suddenly, wide swaths of women had access to two dangerous things—the mail and the post office. Anthony Trollope’s 1852 invention of the pillar-box had given British girls a chance to subvert the authority of their scandalized parents by mailing letters in secret, but their New York counterparts who visited the post office could both send and receive mail almost entirely unmonitored by those who might want to regulate their epistolary lives.
There's more, including the introduction of home delivery, both as a way to protect (and control?) women who might otherwise visit a post office and to receive letters from the many Civil War soldiers away from home. It's quite interesting.
I should warn you, though, that you can spend a lot of time browsing Lapham's Quarterly, since there are all sorts of interesting articles there, from the West Point Egg Nog Riot of 1826 to the really gruesome details of ergot poisoning. It's a real time-waster.