The cost of secrets

A retired chum tells me that posting a letter used to be cheaper if you didn't seal the envelope and only tucked the flap in. Wanting a bit of evidence for this top fact, I googled the following.

"On 1st October 1870, the first official postcards in Britain were issued by the Post Office. [...] The officially produced Post Card carried a prepaid stamp to the value of 1/2 d, a new postal rate for open correspondence. The postal rate for letters in a sealed envelope remained at one penny. At half the standard postal rate, the Post Card was immediately popular, and 675,000 were sold on the first day of issue."

David Simkin, "Seaside photography - the picture postcard"

Wonder when that stopped being the case. Wonder if my legion of readers can supply the answer. Oh, go on. It works for Neil Gaiman.