When Amazon announced in the fall that it would start donating 0.5 percent of some customer purchases to a charity of the shopper’s choice, it was a reason for many nonprofits to rejoice. But the program, dubbed AmazonSmile, included one big hurdle to adoption: Would-be do-gooders needed to start their shopping at the Web address smile.amazon.com instead of Amazon’s regular homepage. Some people I know have bookmarked the new Web address to try to make it easier to remember to shop through the right site. Others have installed a browser add-on that automatically redirects them to AmazonSmile when they navigate to Amazon’s regular site. Still, it seems likely that just as many people at some point made it to AmazonSmile, but have since forgotten altogether about its existence.