Amazon has started their On Demand Plus program. Now when you decide you want to buy a title you have rented, you’ll get $4 credit towards the purchase of that movie, or $5 credit towards the purchase of a TV show. The credit can be used for either Blu-ray Disc or DVD purchases. Current eligible titles include Robin Hood, Letters To Juliet, and House: Season Six. The way it works out, you essentially get the rental free if you go on to purchase that title. Here is the full list of eligible movies. And, here is the full list of eligible TV shows.