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I originally set it up to show amazon every third item, but then I changed that to show every item. Despite changing it twice, I'm still only getting it every third item if that.
And, if I have a post about, say, a terrorist bombing it might insert a link to a book about kittens. You can also choose to display different amazon categories however.
I haven't had much success with amazon and ads in the feed haven't done anything that I can see but then again I don't have hundreds or even dozens of feed readers.
Kanoodle also has a way to add ads to your feed, but I don't know anything about that.