Emma Email Marketing add-on will add an option to the checkout page that allows users to signup for your Emma newsletter at the same time as purchasing. As just a simple check box, 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 Emma Public API Key, Private API Key and Account ID, choose the email list to subscribe users to and an option to define the checkout field label.