You're even understating the case for this particular issue. I own a Shopify store, and one thing that I found out early on is that Shopify does not want you to mess with checkout. It is far and away the most limited piece of their application in terms of customization, and their help articles made it clear that it could not be changed. This app is built on top of a platform and doing something that the platform did not intend for you to be able to do.

I don't begrudge OP making some money on this product, but I'm definitely not sympathetic to this outcome.