One click upsells can increase average order value by 10%-35% or more. The idea is to get people who have already said yes to an offer to buy something else.

Some examples of existing Woocommerce plugins that I found that can do this:

And please do add Braintree integration.

Also depending on how this would be implemented, we should also have the option to skip adding to cart altogether and show the checkout page.

Additionally, we should be able to put the checkout form on a standard product page or a custom product landing page, but I think that may already be possible using the Woocommerce checkout shortcode (still learning all of this myself).