1) The subscription URL starts with http: instead of https: though your site has SSL set up.
2) The subscription URL can only be used by someone who is logged in. To improve onboarding, I think it should go to the create account screen as opposed to the login screen.
For me personally, I won't use (divulge my friend's email addresses to) this software unless it's open source and I can host it on my own server.
I'd be happy to donate $20 if I end up using it.