There is an achievement in The Cave called "Shoplifting". The achievement text says:
Saw a postcard stolen from the Gift Shop.
I assumed it meant the postcard you could pick up from the Postcard Display next to the Gift Shop Clerk.
However, I took the postcard from said display, carried it around with me for quite a while, and never got the achievement. This leads me to believe I either a) did something incorrectly, or 2) mistook this postcard for one later in the game.
How do I get the Shoplifting achievement?