The BUY NOW button is not compatible with the app thus it’s required to be hidden when the products use the options of the app.
Or else, when customers purchase the product from the BUY NOW button, there will be no option of the app on the order page.
Moreover, the BUY NOW button is the default button of Shopify, we can’t intervene or integrate with our app, therefore, the only way to resolve this issue is to hide the BUY NOW button for the products that have the option of the app.
Note: As for the product without the option of the app, the BUY NOW button will be kept intact (the app will not intervene at all)