GoAffPro provides you with the option to integrate third party checkout apps.
To integrate third party checkout apps, go to the Integrations section in the Settings tab of the GoAffPro admin panel.
Here, go to the Third party checkout section.
Now, click on See instructions.
This will open up the Third party checkout setup window.
Now, click on Copy code and paste it into your custom checkout's order status page (or order confirmation page or thank you page).