The country whose 1840 Penny Post helped popularize Valentine's Day cards was the United Kingdom.
On January 10, 1840, the Uniform Penny Post began throughout Great Britain and Ireland. It introduced a much simpler, cheaper rate: a letter could travel between places in the United Kingdom for one penny, rather than costing according to distance, sheets, and delivery arrangements. The reform was strongly associated with postal reformer Rowland Hill.
The new rate made it practical for ordinary people to send paper valentines through the mail. The prepaid Penny Black stamp, introduced in 1840, reinforced the change by allowing postage to be paid in advance. About 60,000 valentines had been posted in the United Kingdom in 1835; roughly 400,000 were sent only a year after the new system began.
The Penny Post did not invent romantic valentines. Handwritten messages and handmade cards existed earlier, but cheap, reliable postage helped transform the custom into a mass Victorian industry. The United States later developed its own large card market, notably through Esther Howland, but the 1840 postal breakthrough was British.