Promotion Codes is a Stripe Billing and Checkout feature which enables you to generate customer facing codes which can be redeemed to apply a coupon to an order. Promotion Codes have advanced validation features which allow you to specify when and on what orders the codes can be redeemed.
This feature is only available for paid Billing users.
To learn more about how to set up Promotion Codes once you have access, check out the integration guide here.