It is pretty clear that Apple is supposed to take money for functionality provided by an app. If you get benefit outside of it (like with Uber, physically transporing you), you may have your own processing service. It is also clear that if you can enable some functionality from outside the app, you can't link to non-Apple payment services from within the app nor you can't mention enhancements. If users somehow discover them on your website, it seems to be ok.
However, with some types of apps the lines are more vague. Say, we have an email client, which works over IMAP, and can work with any email account. And suppose we provide free accounts and 1 GB of space for free, and 10 GB for $1. Naturally, when user gets to 80% of his quota, we'll send him an email, which will be displayed within an app and will link to our website suggesting do a non-Apple payment.
Common sense says that this should be allowed, but I'd bet that apple moderators won't allow it.
Can Apple actually stop developers from sending a payment email to end-users? Would that even be legal, especially if the web-client is a cross-platform solution available on PC, mobile, etc?
That would be significant overreach, ripe for legal action, imo.
However, with some types of apps the lines are more vague. Say, we have an email client, which works over IMAP, and can work with any email account. And suppose we provide free accounts and 1 GB of space for free, and 10 GB for $1. Naturally, when user gets to 80% of his quota, we'll send him an email, which will be displayed within an app and will link to our website suggesting do a non-Apple payment.
Common sense says that this should be allowed, but I'd bet that apple moderators won't allow it.