Tonight I needed a coupon on one product on a site that only sells one product. I didn't really want to deploy and configure Commerce Coupon, so I just did it with a quick combination of a line item field and a rule.
Here's what I did:
- Add a "coupon" field to the line item
- Create a product pricing rule that checks to see that when the right product is in the line item and the right value is in the coupon field, apply a discount. (The rule is attached to this article.)
Here's the screencast.