This add-on will add an option to the checkout page that allows users to signup for your newsletter at the same time as purchasing. As just a simple checkbox, this is guaranteed to create more newsletter sign-ups than a regular subscribe form placed elsewhere on your site.

In the Extensions section of the Easy Digital Downloads Settings, you will have options to enter your Campaign Monitor API key, enter a Client ID, choose the email list to subscribe users to, and an option to define the checkout field label.