Adding our plugin to books is easy - you just upload the book file (using filepicker.io from our batch) and it is automatically modified to include the plugin.
Also, thanks for the feedback about pricing! We're looking into some changes that would address your concerns.
I'm guessing this info isn't on the website because you haven't got around to it yet, but knowing how things work is what attracts me to services. I'm wary of companies that claim to do things but don't say how it works, so you should consider putting up a "How-it-works" page. Do you have a list of supported formats?
Just another comment about the pricing, 500,000 is a LOT of readers. Very few indie publishers (at least single authors) are near that number for a single book. Having a range for authors between 5,000 - 100,000 might be a better sweet spot.
Also, thanks for the feedback about pricing! We're looking into some changes that would address your concerns.