Where to go to pay for an extra test in Study.com?

[bjcheung77]

That's true, you can take it if you're a paying member. Unfortunately, you're not authorized to do so on the scholarship.
If you know you're going to be taking more courses, pay for it through an additional account. That's the only way.
So, pay $199/month for two more exams, and then pay $70/extra for a third, or subsequent exam.
You need to ask yourself, how many courses/exams are you going to be taking with them...