People, naturally, don't tend to download some app to print their photos. In fact, people are not inclined to print photos. So there's an app which people actually use every day, it partners with a service provider to sell a service and get commission out of it.

There are examples like Song Pop game gives you to opportunity buy the music you guessed from iTunes. There's an affiliate problem of iTunes for that. Because Apple (iTunes) itself won't be implementing a game to increase sales.

Here, the point is, people won't print their photos every day. But people will take photos every day. So if I would download Walgreen app and can't find anything to print at a moment, I'll forget to use and print it when I take a picture with Instagram. So if I would see a print button integrated in Instagram, it will dramatically increase sales.