Help finding specific build-a-box or builder app for multi-step user experience

[poopbagclub]

Hello, can anyone help me determine what build-a-box or page builder was used to create this flow: https://www.getfussy.com/pages/subscribe 

I found this website by browsing Shopify's blog here: https://www.shopify.com/za/blog/checkout-process-optimization#1  (#11)

Any help is greatly appreciated!