Federal History journal for April 2025 has a scholarly article by Christopher Shaw about the history of the postal money order. Here’s the abstract:

The domestic postal money order is a neglected facet of the flurry of legislation enacted during the Civil War that expanded the national government. Its introduction extended the Post Office Department’s role beyond serving as a communications medium. The absence of southern legislators following secession who opposed expanding postal services created an opening to secure this reform. As the service matured, postal officials adjusted the policies governing money orders to better adapt them to emerging conditions. By providing a means for Americans to convey small sums of money securely over distances, the postal money order had the broader consequence of bolstering government efforts to promote economic development.

Read the article here.