Terms of Use
Sunflower is free. There is nothing to buy, no subscription, and no account. Use it as much as you like, on as many of your Macs as you like.
The licence
Sunflower is downloaded from this site rather than the Mac App Store, so this page is the whole agreement. You may use the app for free, for anything you like, on as many of your Macs as you like. What you may not do is sell it, redistribute it, or pass it off as your own work.
The app is signed with an Apple Developer ID and notarized by Apple, which is what lets macOS open it without a warning. That is a check for malware, not an endorsement: Apple has reviewed nothing about how the app behaves.
No warranty
Sunflower is provided as it is, without a warranty of any kind. It is careful about what it removes: nothing is deleted without a confirmation naming it, deletions go to the Trash rather than disappearing, and the app refuses to touch system paths. Even so, you are the one deciding what goes, and the developer is not liable for data you choose to delete or for any loss arising from using the app.
Keep backups. That is true of any tool that removes files, and it is true here.
Your files stay yours
Sunflower claims no rights over anything on your disk. It does not collect, transmit or store your data anywhere beyond your own Mac. The privacy policy covers this in full.
Changes
These terms may change as the app develops. The date at the top changes when they do.
Contact
Questions can go through the support page.