Customers who opt-in to email marketing from your WordPress e-commerce site during checkout are added to a specified list on MailChimp.
Currently, there is an option to "opt-in" to email marketing during the checkout. However, this option only provides the store owner with permission to send emails to the customer.
It does not automatically add the customer to any third-party email services.
With Shopp + MailChimp this issue is resolved. If a customer "opts-in" to email marketing during checkout, then their email, first name, and last name are sent to a specified list on MailChimp.