Your customers can pay the remaining balance of their orders from the My Account section of your store by activating the My Account Page Payment Link feature.
Please follow the simple steps below.
Step 1: Open Simple Invoice from your Shopify store’s “Apps” section.
Step 2: Click on the “Settings” on the Simple Invoice dashboard.
Step 3: Scroll down to the My Account Page Payment Link section, and click on the Activate button.
Step 4: If you wish to add additional information such as specify the default value/name of the download button, click on the Message section, add your message to the newly opened window, and click on the Send button.
Step 5: Accept our collaborator access request from the Users and permissions section under the Settings on your Shopify store admin.
Here is an example screenshot of the added feature.