books before paying

Met up with one of my close friends from high school, going to see the film "Sometimes I Think About Dying" together. 

Before the film we enjoyed delicious pho, a Vietnamese soup dish; and after the film, we went to a nearby bookshop, where I found this sign.

This sign was at the cafe in the bookshop. It seems to be telling you not to bring in any book that hasn't been paid for yet; that you haven't bought yet.