In App Purchase

Hello, I have an education application and I want to sell trial exams in this application, or I want people to create exams in exchange for money. Can I use a different payment provider other than Apple's app purchase for this? If I can't use it, is Apple's cut rate 30%? I also want to add this. In my application, users already purchase premium membership packages through Apple's in app purchase service. Can anyone with knowledge help? Thanks