CubeCart has a build-in module for integration with Post Affiliate Pro.
If you have an older version of CubeCart which doesn't include this module, you can download it from here (depending on your version):
Installation and configuration
You should upload this module to your CubeCart installation directory.
After that, all you have to do is to click Affiliates menu in Modules section of your CubeCart admin, and select Post Affiliate Pro.
Once you are in the configuration section, enable this module, and type in your installation URL of your PAP, like this:
Save it and the module should start working immediately. Do not forget to integrate your cart with the click tracking code, so the sale tracking can work.