Zen Cart Manufacturer Discount allows you to offer discounts on products from a particular manufacturer.
The discount amounts are entered into the admin panel using Admin->Catalog->Manufacturers in the "Discount" field under the Manufacturer's Name.
The discount is shown on the second page of checkout by default:
However, using the Discount Preview module will allow you to show the discount on the shopping cart page.
For more information, please visit this product's webpage.