Coupons
The coupon builder can be found by clicking on "Payments" in the left sidebar and then "Coupons" on the top nav.
From there, you can see all "Redeemable" or "Expired" coupons.
If you click on "+ Create Coupon" you can create a new coupon.
There are a lot of different options, such as single and bulk coupons, with options for how frequently they can be redeemed. Auto generated coupons, coupons you name yourself, plus you can choose fixed amount or percentage based coupons.
Make sure you watch the video at the beginning of the help doc to make sure you understand exactly how the coupon system works.
Once redeemed, Coupons are applied to all items in the order.