Discount code module allows you to assign a discount code to a product, category (for each product of this category) and total order. You can specify a fixed discount rate or a percentage of the total order, set expiration date and minimum order amount for a discount code.
Customer gets a discount by clicking a link or inserting the discount code while checkout. The link containing the discount code (http://www.yourshop.com/?discount_code=12345678) is especially convenient for newsletters sending out and promo campaigns conducting.
Screenshots are available at http://high-quality-php-coding.com/.