How do I add discount codes in emails to use as a refund?

As a Shopify store owner, it sometimes really sucks to have to refund clients for products in my collection. Since it's also unavoidable because customers do change their minds, I want to make sure that I provide them with good service so they purchase items again. As a way to reduce the loss in revenue, I decided that I should start adding discount codes for refunds, but I don’t know how to do this.