The Bundler's upsell funnel popup, which allows you to promote the bundle whenever a customer adds one or more items from the bundle to the cart, can be configured for each specific bundle separately. 

To configure it for a specific bundle, simply go into Bundler > Funnels and create a funnel popup for the desired bundle. 

And to change how the popup looks like, simply go into Bundler > Settings > Funnel settings. In there, you can change the text of the funnel popup, colors and other settings.