Lisa Harmon posted these 10 tips to enhance your transactional messages over on the EEC blog:                   

1. Include your company logo and colors to make transactional communications feel consistent with your other marketing materials.
2. Include navigation items relevant to the transaction
3. Use text treatments, color and graphics to maximize usablity and legibility.
4. Make it easy to locate the most critical account and order details.
5. Include customer service contact information
6. Say “thank you.” 
7. Show product photography and link product names back to your website
8. Cross- and up-sell relevant products to already-engaged buyers.
9. Add valuable content and offers.
10. Protect the primary purpose of the message.

Read the full blog post and check out a bunch of examples here.