Use the discount manager tool in Shopify to create a "Discount Code" where customers will enter a code at checkout to receive a discount.
Here is an example code that we are creating that applies to a specific product and discounts a specific amount off the price.
To enable the discount code entry box on your cart page, select one of the three options on the Discount Code Settings page.
When the discount code setting is enabled you will now see a Discount Code box on your cart page. Enter the newly created discount code and click apply. Based on our setting of discounting from discounted price we should see a new total cart price.
Click apply and the additional $2 discount will be applied.
**Please note the limitations listed in the Discount Code Settings page of the application.