Integration with Shopify is slightly different to the norm as they insist that any visitor to your site must specifically agree to receive information (a Newsletter) by email. That means one cannot automatically add them to a List just because they made a purchase.
They do however accommodate HTML Code for an Optin Form being inserted, either on a special Newsletter optin form page, or in every page in your store. A simple “Please add your email to receive our Very Special Offers” Optin Form in the footer of every page, and maybe in the middle of one page, would seem to be a great way of gathering subscribers, and with the advantage that they would not just be those who made a purchase!
The code to use would be the “Basic HTML Form” code available as per the guidance in http://setup.sendreach.com/integrations/basic-html-code-integrations/
The Shopify instructions for doing that are at Shopify Resources