The Shopify POS App SDK now supports applying discount codes
API
Effective November 20, 2018
With the latest update, you can call setDiscount as an Edit Cart action with a discount code that was created in on the Discounts page in the Shopify admin.
This feature is supported for merchants who have Shopify POS Android v3.1.3 or iOS v5.2.1 installed. You can learn more about it here.
We encourage you to try setting cart discounts with a code. We’d love to hear your feedback and how you’ll use this in your app. If you want to make a suggestion about this feature, or if you have any questions about it, then don't hesitate to contact the team at pos-apps@shopify.com.