Valentine greetings go back to the Middle Ages, but it was the advent of the penny postcard that turned Valentine's Day into a multi-billion dollar greeting card event.

Valentine Love tokens are Positively Medieval

In the Middle ages, Valentine greetings were said or sung, but by the 15^th Century beginning to be put into writing.  Sometime in the 18^th Century, Americans adopted the custom of sending valentines that were imported from England.  These were often religious in nature, and some believe that the sacred heart depicted on the cards evolved into the valentine heart, and that the angels accompanying the heart gradually became Cupid. Despite the existence of the pre-printed card, the majority of valentines was one of a kind and made by hand.  Consequently, few exist today.

Love goes Postal

During the Victorian age of mass production, advances in printing techniques made it possible to purchase a high quality, machine made valentine.  However, these were almost always hand delivered because mailing them would have been prohibitively expensive. Around 1870 or so, love went postal, and valentine postcards appeared in the marketplace.  For the first time ever, anyone with a penny in their pocket could profess their love via US Mail.  These penny postcard valentines were wildly popular, and collected in albums, and displayed on mantelpieces. 

Poisonous Postcards-The Penny Dreadful

Earliest valentine postcards were of high quality German manufacture featuring beautiful women, children, flowers, and handsome couples that embodied the virtues of the day, but soon the penny dreadfully and vinegar valentines entered the fray.   John McLaughlin, a New York printer produced a line of badly printed, poorly colored cards on cheap paper that made fun of old maids and schoolteachers.  These were marketed as "Vinegar Valentines" and sad to say, were highly successful.  American Cartoonist Charles Howard developed a line of comic (?) postcards, the so-named penny dreadfulls because the cost a penny both to purchase and post, and were dreadfully mean.  These postcards became more offensive and risqué, and postal authorities rejected cards that were unfit to be delivered.  By this time, greeting card manufacturers were mass producing cards in envelopes that became more affordable to buy and send.  The Valentine Postcard fell out of favor, but not before it played its part in making Valentine's Day the second biggest greeting card day on the calendar.