Ideas to Get You Started
Gain customer feedback by asking them to complete a quick Net Promoter Score survey or answer a single question. A click-to-answer approach would work well here - click here for yes, or here for no.
Once you've identified your happy customers, transactional emails give you a great chance to ask your happy customers to refer your company to their friends and, of course, earn a reward. We've seen great success with this approach at SendGrid.
Allow your recipients to subscribe to your marketing emails with a link/button within a transactional email. If they appreciate the value in your transactional messages, they may be more willing to check out your other stuff, especially at that very moment. Also, you might proactively offer a link to their email settings, so recipients remain reminded of the fact that they control the emails they receive. These things might seem to resemble items required by the CAN-SPAM Act, but there's no harm in taking those safeguards even when they're not mandated.
Followers and Likes
Include a few unobtrusive (and nowadays, almost obligatory) icons to facilitate extra follows and likes on Facebook, Twitter and LinkedIn and Google+.
Include a link to documentation, forums or support page to offer help when the user most needs it. This is a nice gesture and, when implemented with some strategy, can be perfectly timed to fit naturally within certain transactional emails that may be related to support issues. e.g., provide a link to contact your billing department with one click on a monthly invoice email.