It just takes one custy to set off a chain reaction at the Hellmark Card Store.
I don't know how often I've gone to buy a card and there are no envelopes. So I grab one from another set of cards that is the same size, leaving that set one short.
I've seen people pick up a card that comes with a plain envelope and then grab a different envelope from another card just because.
I'll even bet there are people grabbing them to jot down shopping lists that occur to them as they walk around and stuff like that.
You pay for the card but of course, the envelope is free in crusty logic.