@BellStone Any app that uses cart attributes will not work with the Buy Now button. This also includes Currency Convertor apps, any app that interacts with the cart, and apps that use an external checkout instead of Shopify's. Here is a brief article by Shopify outlining this issue: https://help.shopify.com/en/manual/sell-online/online-store/dynamic-checkout/compatibility#apps
This will depend on how the app works, unfortunately they don't have an example store for this app or many FAQ articles regarding the installation.
You should reach out to their support and see if they can give you a clear answer. The app also has a 14-day free trial period to test the functionality out.