I read this story on Paleofuture's site here.
In the early 1900s in America people could, technically, mail their children. However, this fact has been exaggerated by people misunderstanding these authentic early-1900s photographs:
These photographs were meant to be funny. They were not an accurate representation of how one could mail one's child. It's not like you could just slap a stamp on a baby's ass and let it go chill out with the postman all day (although that would be a pretty sweet life of adventure for a baby).
When I say that you could technically mail your children, it had more to do with their classification in transport. Catherine Shteynberg, at the Smithsonian, has clarified:
"Clearly, many were startled and amazed by this photo of a postal carrier with a child in his mail bag, and so for some clarification, I spoke to Nancy Pope, historian at the National Postal Museum. She reiterated the information from the Flickr caption for this photograph: first, that this photo was actually a staged piece, and second, that there is little evidence that babies were sent through the mail other than in two known cases in which children were placed on train cars as "freight mail" as this was cheaper than buying them a regular train ticket."
The blog then goes on to give a specific example of this:
"For instance, when 6-year-old May Pierstorff was "mailed" February 19, 1914 from Grangeville, Idaho to her grandparents house 73 miles away, she was in the care of a relative who worked for the train company. Essentially, it was cheaper to call the young girl "mail" and send her on the train with her relative than buying a full-priced ticket."