Explanation:

https://www.smithsonianmag.com/smart-news/brief-history-children-sent-through-mail-180959372/

One of the most overlooked, yet most significant innovations of the early 20th century might be the Post Office’s decision to start shipping large parcels and packages through the mail. While private delivery companies flourished during the 19th century, the Parcel Post dramatically expanded the reach of mail-order companies to America’s many rural communities, as well as the demand for their products. When the Post Office’s Parcel Post officially began on January 1, 1913, the new service suddenly allowed millions of Americans great access to all kinds of goods and services. But almost immediately, it had some unintended consequences as some parents tried to send their children through the mail.

“Postage was cheaper than a train ticket,” Lynch says.

Finally, on June 14, 1913, several newspapers including the Washington Post, the New York Times, and the Los Angeles Times all ran stories stating the the postmaster had officially decreed that children could no longer be sent through the mail.

You can still send live baby chicks.

7 is nothing compared to the 200,000 shipped on trains to the Midwest as "farm labor" on the Orphan Trains.

while one can no longer USPS mail a babby one can still USPS mail a potato

Life made more sense in the old days. If I package myself right, and want to send myself somewhere, why shouldn't I be able to?

But we couldn't mail a relative's cellphone across the country by next day air after they forgot it here. Found that one out today. The sunchokes to a friend were fine, though.

For completeness - sending children through the mail happened more than a century ago - the one was in 1915.

I only mention this because the graphic showing USPS next to delivery services that didn't even exist back then is misleading.