I forgot to mention - the user can pay $0 for any app. This is the problem which needs to be fixed. All apps are donationware, enforced by the appstore.

This is exactly why I never bothered. Combine that with the fact users can pay $0 and then come to GitHub to complain (i.e. create a support burden), I figured it wasn't the best option for a solo developer to build a business model on.

It would be more interesting if there was more flexibility in the monetization. Options for a minimum price + pay more per seat (for integration with some corporate provisioning software), subscription pricing, one time price + either support subscription or support "piecework" (like bug bounties or developer consultation), and invoicing integration.