Amazon tries to encourage this by using a "delivery day" for all your weekly deliveries to come. They need to incentivize it a bit harder I think with free digital content. If I could rent a movie from prime (not something new but let's say original Matrix) in exchange for using the prime delivery day option then I probably would. Otherwise they are relying on people to realize that constant deliveries are a problem and then sacrifice existing conveniences they have for that problem.