Justman if you are running a forum for a non-profit or other kind of charitable or open-source project, feel free to reach out via the email in the documentation for a special license.
On my most complex extensions the fee not only covers the work that went into the current version, but guarantees ongoing compatibility and support, which would expire after a while with a "lifetime" (rather "one-time") license.
2024 was a slow year for feature updates with my extensions, but I have updates in the pipeline and 2.x compatibility will be available right away when Flarum releases it.