In 1797, a British publisher issued The Young Man's Valentine Writer, which contained scores of suggested sentimental verses for the young lover unable to compose his own. Printers had already begun producing a limited number of cards with verses and sketches, called "mechanical valentines." Paper Valentines became so popular in England in the early 19th century that they were assembled in factories. Fancy Valentines were made with real lace and ribbons, with paper lace introduced in the mid-19th century. In 1835, 60,000 Valentine cards were sent by post in the United Kingdom, despite postage being expensive.
A reduction in postal rates following Sir Rowland Hill's postal reforms with the 1840 invention of the postage stamp (Penny Black) saw the number of Valentines posted increase, with 400,000 sent just one year after its invention, and ushered in the less personal but easier practice of mailing Valentines. That made it possible for the first time to exchange cards anonymously, which is taken as the reason for the sudden appearance of racy verse in an era otherwise prudishly Victorian. Production increased, "Cupid's Manufactory" as Charles Dickens termed it, with over 3,000 women employed in manufacturing. The Laura Seddon Greeting Card Collection at Manchester Metropolitan University gathers 450 Valentine's Day cards dating from early nineteenth century Britain, printed by the major publishers of the day. wikipedia