What is the origin of this tradition?

Explanation

Christmas Seals are labels placed on mail during the Christmas season to raise funds and awareness for charitable programs. At the beginning of the 1900s tuberculosis was a greatly feared disease. In 1903, Einar Holbøll, a Danish postal clerk developed the idea of adding an extra charitable stamp on mailed holiday greetings during Christmas. The money raised could be used to help children sick with tuberculosis. The plan was approved by the Postmaster and the King of Denmark (Christian IX).