Sign up for a Partial.ly account at https://partial.ly. From the Partial.ly merchant portal, click Settings then integrations. In the Shopping Carts section, click the Connect button next to Shopify.
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On the next page you will need to enter the domain name of your Shopify shop (only the portion that comes before .myshopify.com), and then click the Connect to Shopify button

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After you've connected your Shopify account, you'll want to review the settings for the Partial.ly integration with your Shopify account. See our Selling with payment plans on your Shopify website for more details.