Interesting product, good review!
I can understand the point about why cardboard would not work due to thickness. I'm a bit curious if the backing is also too thick to transmit light well.
While it would make the sign tooling a bit more complicated, one path might be to make a slot on one end of the frame that the ads could be slid in and out of to let them be more easily changeable.
Of course for the reasonable price point, the point of the multiple stickers is probably not to be changing them often (though it didn't look too hard to peel the one off), just to make it flexible so the user can pick what sign they want one time during install rather than offering a whole bunch of different billboards.
-Dave
