In my experience, you couldn’t just setup an account and start selling, you had to contact their sales team and they let you know if they want your business.
Last I checked, Polar was using Stripe under the hood.
EDIT: Yup. Via their pricing page:
Polar is currently built on Stripe, and we cover their 2.9% + 30¢ fee from ours. However, they impose a few additional fees for certain transactions that we need to pass on.