Would like to use this in a manner where someone would pa…

HD Quiz is not an eCommerce platform, but there are certainly ways to integrate it.

I think the best way to do it is to create a new “membership” type – a user access level that will be able to see the quizzes. This is really the best way to secure the quizzes so that people can’t just share the link or something.

Take a look at WooCommerce Memberships plugin. With it, you can create a new membership “product” and then create a new page with the quiz on it. This new page should be set to private so that only logged in users can view it. It also looks like that plugin can restrict page content based on if the logged in user bought a certain product – which is exactly what you need.