Something that I came across while researching Christmas in the 1920s was the use of Christmas seals.
Christmas seals are labels placed on the mail during Christmas to raise money for charity. They look like postage stamps, but have no value and so have been called a “cinderella stamp.” They began in Denmark in 1904 to raise money for children with tuberculosis and were brought to the United States in 1907. First administered on a nationwide scale by the National Association for the Study and Prevention of Tuberculosis (NASPT) and the American National Red Cross, by 1920 the Red Cross withdrew, leaving it to be conducted by the National Tuberculosis Association, as the NASPT had been renamed. Eventually the organization would become the American Lung Association in the 1970s and continues to issue Christmas seals to this day.
Obviously many people must already know about these, but they were new to me. It looks like one can still occasionally find them listed for sale on eBay as well as art inspired by them on Etsy. Like most things in the 1920s, I love these designs.