I discovered the FastSpring Examples page the other day. There, you can interact with different store setups to get a feeling for the capabilities of FastSpring’s new and updates Store Builder API. The examples also either include or link to documentation pages and CodePen-like source code demos.
All in all, I think this is a very cool addition to the docs.
When you play around with the examples, you will see that the new store styles aren’t just for fancier checkout pages. The new store has an API that you can easily (!) use to transform even a static HTML website into a dynamic store front, including a live shopping cart. This is very, very powerful.