Warranty Upsell

[FloweringTrees]

Hi,

I was wondering if anyone knew of a good app which can upsell a warranty preferably on the checkout page. It's not as easy as just adding a "warranty product" because the warranty itself needs to be a percentage of the total cost. Does this sound like it needs to be custom coded?

This webpage has a good example of what I am looking to do.

Thank you!