Discount codes at checkout
To include discounts in the invoices sent out by the app, you have 2 options
1) Your Shopify checkout has a discount code field for any custom discount codes that your store supports
2) You can apply a global discount to ALL draft orders created through the app. This isn't done through a discount code but is a global % of $ amount off for all draft orders created through the app.
Please note that using Shopify's "Automatic discounts" feature will not work with SoldLive invoices: