Set the email address session:
1) open the confirm page.
2) go to window -> Server Behaviors
3) Click the plus button in the server behaviors panel and select eCart -> General -> Set Session Value.
4) Set the trigger to any form post
5) Set the name to "EmailAddress" without the quotes.
6) Click the lightning bolt next to Value and select the email address form element.
7) Add Universal Email to the checkout success page, use the EmailAddress session variable for the to address.