I plan to distribute an app on different platforms including iOS (also Android and web).
It will be available only in EU countries, so alternative payment options should be allowed, according to Guidelines 3.1.1(a).

The app will display local events and activities.
Users will be able to add events by filling a form, then the event data will be reviewed, and if validated,
a few days later, the user will get an email with a Paypal purchase link to complete the event publication.

Since the payment process is disconnected from the app
(I don’t even know if the data comes from iOS or Android or website),
do I need to worry about Apple In-App Purchase Guidelines?

And even if I need to use StoreKit External Purchase Links,
how would I proceed since the payment link is in the mail and not in the app?