Helge Heß published an Open Source component to use the modern FastSpring web API so you can display a sheet to purchase a license of your app within your app.

It’s not yet a native in-app purchase UI or anything, but you can

• display your product from your FastSpring store,
• process orders in the sheet, since it’s just a WKWebView, and
• automatically activate the app after the order is completed.

The last part is probably the most important to me. It’s what I like most about the

Check out FSCheckoutSheet on GitHub: https://github.com/ZeeZide/FSCheckoutSheet/