Itch is a store. The items are not supposed to be some mail attachements. And the mail feature is to be used only for notifications and best practise is to not use it all, if at all possible. People that want notifications from you, do follow you. And if you send them emails they can unsubscribe from those mails.
How do we reward those who paid?
By having content accessible for those who paid.
https://itch.io/docs/creators/pricing#purchase-tiers-through-individually-priced...