Apple’s rules prevent vendors from linking to external service account setup/management. You cannot even mention the existence thereof, let alone link to it or advertise the options provided therein. Consumers are explicitly kept in the dark about any method other than payment through the App Store.
That’s the stunning uppercut. The size of Apple’s fee is merely a follow-up kick to the nads.
This is why service providers, from Hey.com to Netflix, have a special irritation for the App Store rules, and since this rule directly distorts markets by affecting consumer choice is why so many competition regulators have a file open about it.