From a customer's perspective, ninkik can be more or less involved. It depends upon the e-commerce platform you are using:
Your e-commerce platform uses ninkik's API to retrieve all allowed products their releases of a customer. When the customer clicks a link, the e-commerce platform asks ninkik to create an access link and will show it to the customer. This method should be preferred if you want a one-face-to-the-customer solution.
Your e-commerce platform redirects to ninkik after a sale has been made. This happens for the Gumroad platform. ninkik handles the listing of all product and their releases. You probably want to use this if you need a low-cost integration into your platform.