How can I add an option to use a shipping promo code to save on price of shipping?

Hello! I am trying to figure out add an option to apply a shipping code during check out to save on shipping. I've added the shipping promo code, but I don't know how to take off the cost of shipping when this option is selected. Thank you for your help!

If you want to leave the shipping charge optional, the only way would be disabling the Shipping option, and creating a new product that would work as the shipping cost:

Let us know if you have more questions.