Are there ongoing costs after a Shopify app launches?

Yes, always. The Apple Developer account renews annually and the Shopify subscription must stay live — if the store subscription lapses, the app stops working, because the catalogue and checkout it relies on are gone.

That last point catches people out. The app is not a separate product that survives on its own; it is a front end onto a Shopify store, and it lives exactly as long as that store does.