A proposal can look perfectly reasonable until you change one option. The price updates, but the delivery time still belongs to the old plan. Now the page is giving two different answers. That is the small problem this example tackles: two packages, two revisions, and one consistent summary. It uses ordinary HTML and JavaScript. The company details and estimates are fictional. The useful part is the shape of the state. You can reuse it for a quote calculator, a package comparison, or a pro...