Ditto what pigeon said. If your email subscription numbers are high, you might want to use a free file host service, to save on your site's bandwidth.
I think you can do password-protected posts on wordpress, which your site looks to be on, if that appeals to you.
On sites where I've seen this, the post won't be entirely invisible (the content of the post won't appear without the password, but people without the password will still see the title of the post). It would say something like 'Password protected post: Free download of x book'.
You'll probably get better feedback on the logistics of uploading to vendors in the
author section of this forum. I could be wrong, but I think you can't have a permanent $0.00 price on Amazon and B&N.