You can create preset discounts to be used at the check out of an order. The names can be customized and be either dollar amount or percentage discounts.
1. Go to the Back Office under Marketing then select Discounts
2. To create a new Discount go to the top right and select New
3. Then you will give the discount a name and either a percentage or a dollar amount that you determine for the discount. Once you are finished hit Save.
When the Discount has been created you will see it appear on the iPad and be able to apply it to the cart.
For more information on discounts see here