We never get postcards at my counter (the circle look book for Barbie was the only one I've ever seen in the 1.5yrs that I've worked there)...
Sometimes they have them at the freestanding stores, but I seem to get most of them in the mail. For some reason I think it all has to do with how much $ you spend at a particular location, but I could be imagining that.
They also are included in online orders when available (but I've never ordered online so I wouldn't know about this).
Honestly I don't see a use for the cards, personally (i have too much clutter as it is). I hold onto the really cool ones or the ones w/ looks in them, but end up giving away the others.