Seemed so promising …

I really wanted this plugin to work, but the custom fields feature does not function properly. Beware! If you use anything other than the default MailChimp merge fields, you will probably have trouble too. I contacted the developer twice (once on 10/02/2018 and again on 10/15/2018) using his preferred contact method. Now, 20 days after my original message, I still have not received any response. Seeing as how the plugin has not been updated in the past 8 months and the author is missing in action, I suspect that this plugin has been abandoned.