Amazon's Prime program is where you can get unlimited 2-day shipping on Amazon products (as opposed to products sold by Amazon merchants) by paying a yearly fee of $79. They recently offered me a one-month free trial and I have already made 3 - 4 purchase over the past couple of weeks. Granted, I have a lot of travel coming up (about which more later) for which I'm stocking up on things but there's no denying the lure of Prime! Prime essentially makes the transaction costs feel free. Yes, Amazon has Super-Saver shipping but that can take up to a week and you need to buy at least $25 worth of goods. With Prime, the 2-day shipping shortens the gratification window and so I've been using it not just to make less commonplace purchases like a new digital camera, but also sundry items like razor blades.
Prime was introduced in '05 and investors hammered Amazon for it but the Company claims Prime is behind their latest revenue increases, which have boosted the share price. I believe it. It's happened to me.
Interestingly, Zappos is a big believer in free, overnight shipping and credits this with a lot of their success. I read an article (forgetting which magazine), in which their CEO mentioned that they invest in free shipping and customer service (as opposed to advertising).