Combining discount codes to work with volume discounts can be accomplished by enabling Discount combinations on the Storefront -> Global settings section

If you want a discount to apply in additional to a volume discount the only way to accomplish this is to create an "Order Discount" in shopfiy.

When setting up your discount in shopfiy make sure you set the ability to combine.

Here you can see the discount applied to the product and the additional discount for free shipping shown as well.