CartFlows Pro offers one-click upsells when using PayPal and Stripe.
With PayPal, it will require reference transactions to be enabled.
One-click upsells needs exclusive compatibility built with each payment gateway and we will add support for other payment gateways one by one.
We're also thinking of a hybrid solution, where the upsell won't be “one click”, but still elegant and conversion optimized.
For example, when users click on the upsell link, a beautiful popup will have a form with required details to proceed the payment for upsell, and this will work with all payment gateways without having us to build any exclusive compatibility. This is just an idea right now and we plan to explore this further in a few weeks.