I got a giftcard to Amazon for Christmas and everything I've ordered is eligible for the "Free Super Savers 2 day shipping" if i sign up for the AmazonPrime free trial. The trial is free for a month and you can cancel at anytime, but if you don't cancel you get charged $79. Is it a good deal to apply for AmazonPrime for the free shipping? Can I cancel right after I place my order?