Option: Manual Conversion + Sendy/Mailchimp + Stripe
Content could be converted using Sigil+Calibre or pandoc+Calibre. Sendy is self-hosted email sending software, Stripe for payments.
Pros:
- Ongoing fees are very small. On subscription payments, 2.5% + $0.30. Outgoing emails are $0.10 per 1,000 on Sendy; Mailchimp is free up to 2,000 subscribers, after that it costs between $8-$20 per 1,000 subscribers (depending on the plan, up to 10k subscribers)
- Complete control over formatting
- Fully branded experience
Cons:
- Potential dealkiller: it's unclear to me whether any of these services allow you to attach ePub/mobi files to your mass emails.
- Managing subscriber payments, device preferences, support requests, could be a nightmare
- Manual conversion of content could easily require adding several fussy steps to publishing workflow.