Coupons Option
Administration have an option to create a coupon that can be redeem on the checkout process.    
This will allow you to offer a promotion coupons to a group of users to promote your site and generate sales.    
You can create as many coupon as you wish and coupon can be set as Amount that will be deducted from the order total, or as percentage.    
If coupon was set as amount and order was backorder, this amount will be applied only once. If coupon was set as percentage, it will apply to any backorder from original order as well.    
Coupon can be set with starting date and expiration date plus it can also be tagged as active Yes/No