Earlier quoted context omitted.
It's not an issue that [insert digital good here] is cheaper on the seller's website than in the Apple App Store. What Apple prohibits you from is telling your client that the [insert digital good here] is available for purchase somewhere else or that there's a special kind of a markup on the App Store option.
I don't think that's entirely true. Apps can use SMS or open the OS browser to accept payments and not be in direct violation of the guidelines.
Which apps are allowed to open the browser for payments? I don't think even Netflix isn't allowed to do it. They just say "You can't make an account in the app" (paraphrasing) on the sign in page.