My wife and I were shopping at Barnes & Noble bookstore this weekend and when we got home, I noticed in addition to our receipt and books, our bag contained a short slip printed after the receipt that gave us 4 recommendations for other books, based on our purchase.
I am not sure how long they have been doing this, but it is a great means to engage your customers further. The recommended books were not extraordinary (they were on the same level as the recommendations you receive from Amazon), but as their algorithms become better, this will be very helpful. Moreover - why not allow you to scan the book you are interested in the store, and have it give you a list of available books (rather than waiting until you've completed browsing and are checking out.)
The reason I go to a bookstore is to buy books that are in stock (I can always order online if not) - so give me those choices!
The same process could be used by smaller retailers as their POS systems become more sophisticated. If I purchase the same products regularly, why not offer me the option to purchase additional items that are liked by people like me.
The unobtrusiveness of the paper in the bag is also nice since I can choose to review the titles or not (without being "sold" on the books).