WP Simple Pay Pro provides many ways for developers to tap into the code and add their own customizations. We use the native WordPress hook system for both actions and filters to allow for these modifications.
If you need help adding custom code you can read our article about adding custom functions.
You can also see specific examples we have created in our code snippet library.
Please contact support for information about specific actions/filters. We highly recommend Codeable for WordPress customization and consulting.