Zen Cart permits you to add discounts to specific types of orders.
Coupons - Create discount coupons for price reductions and free shipping
Group Pricing - Offer discounts to groups of customers
There are also plugins which allow you to discount and reduce the order total.
The discounts described above are order total discounts, which means they reduce the order total. There are also product discounts which reduce the price of an individual product. Take a look at: