AFAIK, for IOS, if you want your users to pay via 'web only', you cannot mention anything related to external payment within the app, you cannot have a payment flow within the app.
What you can do is, ask users to sign in and/or visit your web page from where you can convert users. You can run email campaigns etc. Or if your users already have a premium login by making payment elsewhere, that should be fine.
Pandora (for Pandora One) is one. You can pay monthly through the app, but the (cheaper) yearly option is only available on the website. They can't mention the website option or link directly to it from either the iOS or the Android app due to each market's rules.
AFAIK, for IOS, if you want your users to pay via 'web only', you cannot mention anything related to external payment within the app, you cannot have a payment flow within the app.
What you can do is, ask users to sign in and/or visit your web page from where you can convert users. You can run email campaigns etc. Or if your users already have a premium login by making payment elsewhere, that should be fine.