Who invented an inland letter?

The concept of the inland letter, which is a type of postal service for sending letters within a country, was developed as postal systems evolved over time. While no single individual is credited with its invention, the establishment of organized postal services in the 19th century, such as those initiated by countries like the United Kingdom with the introduction of the Penny Black stamp in 1840, facilitated the use of inland letters. Various postal reforms across different nations contributed to the standardization and accessibility of inland letter services.