

I’m not sure about the legal intricacies of it, but there is commercial software being distributed through flatpak on Flathub for a while now. The first example that comes to mind is Bitwig, a well-known, paid, commercial Digital Audio Workstation: https://flathub.org/apps/com.bitwig.BitwigStudio
Also, Flathub is working on offering paid apps: https://news.itsfoss.com/flathub-paid-apps/
Thanks!