Support
For a question, a problem or an idea about the app.
Write to support@terzatorre.app. What you send is used only to answer you: what happens to support emails, and how long they are kept, is on the Privacy page.
For a problem, three things help: the version of macOS, that of the app (menu Terza Torre › About Terza Torre) and what you were doing when it happened. If it concerns a game, attach its PGN: from the list, right click on the game and Export….
How each window works is explained in the documentation.
The subscription
The Pro subscription is bought in the app, from the Terza Torre Pro window, and managed like any Apple subscription. With Pro active, Manage subscription, in the window or in Settings › Pro, opens Apple’s page where you cancel it; or, in the App Store, click your name at the bottom of the sidebar, then your name again, Subscriptions and the subscription. Apple explains it in Cancel a subscription from Apple.
On another Mac with the same Apple Account, Restore purchases in the Terza Torre Pro window finds the subscription again. Payments and refunds are handled by Apple: the developer does not see your payment details. See also The Pro version.