The “Pay online and get 15% off” button is the default “Buy now” button from Shopify, but renamed.
If you install our app Releasit COD Form & Upsells you can contact us from there and we will help you setup the COD button and the prepaid button with discount!
You can install the app for free from the Shopify App Store here: https://apps.shopify.com/releasit-cod-order-form
(You can also find an example of how this works on our demo store on the App Store)
If you have any questions don’t hesitate to ask!
Marco from Releasit