According to a website on Valentine history the first valentines were either sung or verbalized in the Middle Ages. In the 1400’s the first written Valentine was recorded and can be found in the British museum. It was written romantic sentiments to Charles, Duke of Orleans’s wife while he was detained in prison.
A booklet known as the “writer” came to America in the 1700’s. This was a booklet of poems and verses that could be copied onto fancy paper to make valentines for a loved one. Some would contain return messages that the “sweetheart” or woman would return to the man.
Paper valentines became popular in the early 1800’s in England. This is when they were manufactured in numbers and sold. Esther Howland, an artist and printer, began making American Valentines in 1850. A popular valentine in America in 1860 was the “ Daguerreotype” . This was a card with a tintype likeness in the center surrounded by a wreath. During the Civil War, cards with flaps like tents were popular with a soldier behind the flaps.
In the Victorian Era, when the postage to send a letter was a penny, more valentines were sent by mail. They had previously been hand delivered.
I have a short flash fiction Valentine story at http://romancefromthegenre-istas.blogspot.com/ I did the above research to write the 1500 word flash fiction short story titled Heart of Gold.