Amazon's trial periods must be as quick as its shipping -- just days after we first learned that it was testing monthly subscriptions for Amazon Prime, the online retailer has already put a halt to the program. A spokesperson says only that the company has "completed" its testing and has stopped taking sign-ups. We haven't been told whether or not the test was successful, although it's not hard to see the math working against Amazon's experiment when a $79 yearly Prime subscription is unquestionably the better deal. Chalk up the rapid-fire testing to the competitive heat in the streaming video world.