From the Smithsonian's Flickr stream of historic, public domain photos, a shot commemorating the end of being able to ship your children by postal mail:
This city letter carrier posed for a humorous photograph with a young boy in his mailbag. After parcel post service was introduced in 1913, at least two children were sent by the service. With stamps attached to their clothing, the children rode with railway and city carriers to their destination. The Postmaster General quickly issued a regulation forbidding the sending of children in the mail after hearing of those examples.