On the eCart support page on the right side there is a link to archived documentation. If you click on this link to bring up the archived documentation you will see a section titled:
Solution Recipes (eCart 4)
In here is the solution recipe for coupons, it is titled:
Creating a Coupon system for your eCart store (Interactive Tutorial)
Printable Version (PDF)
This is the documentation that we have for adding a coupon code to your store. Please give this a try and let us know if you have any questions.