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¢

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.