There is a cute series by Charles Twelvetrees that shows two children and the theme is about writing a letter. The series is published by S. Bergman .
The above card shows a boy bandit saying “Hands up! Come across with that letter you promised to write.” The message on the back simply says “Well-” and was mailed in 1916. This is my favorite one it is so adorable.
“I can’t express in writing how I feel.” Mailed 1917.
“Write your answer real soon.” Mailed 1920.