I've written a very small extension for Safari.
It redirects any Amazon URL to the "smile.amazon.com" page so that a portion of your purchase price is donated to the charity of your choice. My charity is the First Tee of Pittsburgh.
You can download it from there. I can show anyone the code if they have any doubts as to the nature of the extension. If you trust me enough to go by this picture, though, you can see that the only pages that this extension can act on are here:
Again, download it from my tweet: