update your list with each new customer purchase. To get started go to account ->integrations ->e-junkie.
For more information: http://kb.mailchimp.com/article/how-does-mailchimp-integrate-with-e-junkie/
I can't get ejunkie and mailchimp to talk at all. Cant get address data pulled over into MailChimp even if I could, because it wants me to have a field for 'street.' The address field in the signup forms has one field for 'street' 'city' 'zip,' etc. Does this work for anyone?