Issue:
Users that have an active subscription through Apple's App Store cannot add payment method in Proton Mail settings. When they click on "Add credit / debit card" they are greeted with the "Manage your subscription on Apple App Store" popup.

While I understand why it wouldn't make much sense to have a payment method on an account that has their subscription managed by a 3rd party, I've encountered an edge case that I think would be beneficial to the <1% of users like me.

I've cancelled my Proton subscription through my Apple Account. Now, since the Proton mail settings don't allow me to add a payment method, I need to wait for my subscription to expire, then add a payment method to Proton, then pick a subscription.

I just think this would be beneficial to implement, don't see any issues with adding a payment method since the user can be notified that the payment method is inactive or will not be used as long as they have a 3rd party controlled subscription. But any additional purchases or activations through Proton can still be completed with the added payment method.