Most of our customers will be using Mailchimp Pro for selling access to paid newsletters. The purchase process is an implied consent to be added to the newsletter. So, we don't currently offer double opt-in out-of-the-box for the MoonClerk payment forms linked to Mailchimp Pro.
However, you are still able to use double opt-in. You'll just need to enable double opt-in in your Mailchimp account. Once you do that, your payers will receive an email from Mailchimp for them to opt-in to your email newsletter.
If you would like to see double opt-in offered out-of-the box with the Mailchimp Pro extension, we'd love to hear from you about that.