I've looked into this for you and i think I have your solution. In your main "amazon-product-in-a-post-plugin" folder you have the file "amazon-product-in-a-post.php".
Open this file and do a search for
and replace that with your no follow,
href="' . $result["URL"]
rel="nofollow" href="' . $result["URL"]
I found 4 of these matches, one of which was commented out.
I've tested this on a local copy of wordpress and it's working for me, I tried both editing an existing post and from creating a new one. I've not looked at every option configuration but this should be the answer.
Let me know if this was your solution.