Awesome, thanks so much for your reply @tobebuilds! Hoping this will actually be the case. For my peace of mind, it would be great to get an 'official' confirmation from someone at Shopify, but this is already very helpful.

Hi there, We're planning to build a Shopify app that will be completely free. No charges at all, no paid plans or anything. In the Shopify requirements, it says that apps need to use the Billing API "unless you've been notified otherwise by Shopify" ...