Paid apps in Ecwid App Market
Many paid applications are created by 3rd party developers. Usually they require a monthly subscription and give you a free trial so you can start right away.
The billing model varies from app to app, some applications can have multiple pricing plans to better fit your needs. When you are just starting, it makes sense to use a basic plan and learn more about the software first. As your system grows, you may want to upgrade to a higher plan where more features are available.