Katy here from Shopify. Thank you for reaching out, and great question.
At present, it is not possible to combine or stack discount codes together, we have a little more info on this here.
While there is an app that allows you to do so, this does not work with shipping.
However, as you are looking to combine a % off a product along with free shipping, there is an option which may work for you.
This works by setting up Free Shipping at the checkout, without the need for a discount code.
You could perhaps have this available if a customer orders over a certain amount, for example. There is a guide on how to set this up here.
I hope this helps! I have also passed this feedback along to our team as a feature request.
The app you are referring to has only one review, and you cannot combine a % discount such as 15% off your first purchase, with a black friday discount of $30. Seems to only work if both codes are % or both are $.
Seems to be a lot of interest in this. Why can we not have the option to allow two codes to be used on one order? The higher discount % or $ should always apply first.