If you are using an Online Store 2.0 compliant theme, you can use the following steps to setup your upsell widget.
At the Theme Customization page select Add Block:
You are required to setup 3 components for the Upsell+ app:
Upsell and Gift - This sets up the widget displaying the volume discount, mix and match, free gifts and product recommendation.
Pop-up Upsell - This will enable the pop-up upsell.
Frequently Bought Together - This sets up the widget displaying the Frequently Bought Together feature.
To setup the components, select one of the Upsell+ components and then proceed to position the widget based on your desired location.
Repeat this with the other 2 components. Regardless of position, the pop-up modal will only be displayed based on the settings made in the app.
Lastly, save the changes you had just made by selecting the Save button.
