1-Click upsell functionality is automatically integrated into your sales funnel. However, there are a few prerequisites to ensure true 1-click upsell functionality is in place.
#1. You must be using a payment gateway that supports 1-click functionality.
*Update: You can now use PayPal(Beta) to enable 1-click upsells with Paypal and PayKickstart.
#2. Make sure you connect the appropriate Payment Gateway under Campaign settings.
#3. Ensure that you link your Buy Button on each sales page with the correct link found in the Funnel section.