It's true.  It used to be legal for people to send kids in the mail.  When the United States Postal Service was first introduced, Americans were able to send packages across the U.S. at an affordable rate. 

But of course, there were some early abusers.  In two cases, people actually put a stamp on their kids so they could be sent through the mail.  It didn't take long for the Postmaster General to issue new regulations banning this practice. 

Above is a classic photo from the period of this practice.