Smashwords has the option for the author to specify "you set the price", so that's an easy way for you to do your donations plan.
Smashwords has two other advantages over Amazon.
First, if the author revises the book you can download the new version. With Amazon, you're stuck with the version you originally purchased. Now this makes sense with printed versions, since there is cost associated with printing the second, third, etc. editions, but digital? It actually probably costs Amazon more to keep track of the version any individual purchased and maintain that copy on their servers, than to simply keep one version of any given book and let anyone who purchased it download it/read it.
The second thing is that if you read deep into the ELUA for Amazon, technically you are renting the book, you don't own it. So should they get cranky with you at some future date, they can remove any given book from your library.
That's one reason I download everything, and put it in my Calibre library.
