Individual custom fields email merge tags have been implemented in test mode.
You would be able to use them in emails using {custom_field_N} email merge tag, where N is custom field’s ID (you can see field IDs on Stripe Payments -> Settings, Additional Custom Fields tab).
If you want to help test these, hit me up here https://s-plugins.com/contact-us/ . Just put “ACF testing” as a subject, I would understand what this code phrase means 😉