Memberful

Memberful handles the membership and paywall layer while staying out of the way of whatever you're already using to send email — it integrates with WordPress, Mailchimp, Kit, and others rather than replacing them, which suits writers who don't want to migrate their whole setup just to start charging for access.

It charges a flat percentage on top of Stripe's own processing fees rather than a monthly subscription fee, which tends to make sense once you have a meaningful number of paying members, but is added overhead before that point.