Support
Write to support@menubuddies.app. A person reads it. The quickest way is Send Feedback… in the duck's menu: it opens an email with the app and macOS versions filled in. If the app quit unexpectedly, it offers to email a crash report the next time it starts.
Restore a purchase
Open the duck's menu and choose Restore purchases. Apple restores the unlock for the Apple Account that bought it, on any of your Macs.
Refunds
Apple handles refunds. Use "Report a Problem" on Apple's site.
Uninstall
Quit the app, then drag it from the Applications folder to the Trash. Its settings and battery history are small files in the app's container in your Library folder. Your purchase stays with your Apple Account.
For everything else, see the FAQ on the home page and the privacy policy.