So I wrote a book on beekeeping a year or so ago. I sell it on Amazon but also via Clickbank. The trick was to protect the delivery page from bots and google spiders and other folks who wanted to download the book for free.
There was an old solution that an update to WordPress made invalid, I posted about it in an earlier post.
For those who want to send someone a url to a password protected page without them having to log in, this is it. It is called the “Post password token” plug in available for free on WordPress. But give them $10 donation if you use it, for good karma.