Add a pay another way option during the upsell process, where they can switch their payment method. This is helpful because some people don't have the funds available on the initial payment method when they want to purchase an upsell. They didn't know about the upsell so they weren't planning on spending more when they chose to use thier inital payment method. Other checkout systems (ex Thrivecart) offer this option.

I have attached an example screenshot of what I am requesting.