Customers who opt-in to email marketing from your WordPress e-commerce site during checkout are added to a specified list on Mad Mimi.
Visit madmimi.com to register for an account.
Shopp is an ecommerce plugin for WordPress. Learn more about Shopp.
Yes, there are different extensions available including emailing marketing, short message service (sms) notifications, forms & reporting, help desk, and more. Please visit Shopp 101.