Get Emails automatically tracks visitors on your site and cross references multiple data points to get their email. It's completely within CAN-SPAMM act guidelines.
But with my short experience with Cart Stack it seems you are open to creating/implementing cool and different features.
If there could be a way to track visitors with an integration with Get Emails, it would be getting more email addresses with exactly what they browsed and did to send them a relevant message, that could be a huge win.
CartStack customers get "free" access to Get Emails which further expands the effectiveness of campaigns.