To achieve this functionality you will need to integrate a payment processing platform into you app. Then you can initiate the the payment process when a user clicks the order now button.
There are several existing marketplace connectors that should make this process easier for you. Here are just a couple of them:
Hope this gets you pointed in the right direction.
You needs to use any one of payment gate way to achieve the payment
In addition to the answers specified, make sure you have your security covered. Mendix makes it very easy for you to build apps but you need to be very carefull when working with data like this. At the very least go trough this learning path: https://academy.mendix.com/link/paths/9/Configure-Advanced-Security
In the case this is a PoC just show a page with some logo's of payment providers and create mock thank you and error pages, save yourself the trouble of creating the actual integration.