Can I use third party payment system in my app?

Hi, I'm developing an application with Flutter and it will work on both IOS and Android platforms. My app includes subscription(1-3-6 months, 1 year) and in-app purchase for content which is like udemy courses. User can list courses and purchase one of them. My question is related with that.

  1. Can I use third party payment system for subscription?
  2. Can I use third party payment system for purchasing item like course ?