One sketchy thing I notice right off the bat is that they're only doing this if you go to smile.amazon.com
each time you make a purchase. So even after you register and select a charity, if you forget to go to that specific URL your charity won't be receiving any funds. I'm sure they're banking on a lot of goodwill and positive press from this announcement, followed by most people forgetting to enter that special URL each time they shop. If Amazon was serious about getting into customer-driven charitable giving, they wouldn't be adding these unnecessary extra hoops to jump through and remember forever.