I have seen this type of behavior before on options expiration and when they are about to issue more shares.
I have Prime and use it for shipping about once a week on average, saving probably $250 a year on shipping. $20-40 will not phase me at all. If I were Amazon I would also be looking to charge Netflix more for server use.
Per this article, http://seekingalpha.com/news/1530371-amazon-reportedly-prepping-sub-300-set-top-game-console