Gift Box allows you to create different types of gifts, and you can use them according to what you need. One of those gifts can be Free Shipping. It will surely motivate your customers to spend more in your store and can bring you more sales.
To create a free shipping gift, first, you’ll need to create a free shipping rate in your store. In your store's Shopify admin just go to Settings » Shipping and delivery » Manage Rates » Add rate, and enter the Minimum price for Free Shipping (the total amount you want your customers to spend in order to get the free shipping):
Once you have created the free shipping rate on your Shopify admin store, you can go to the Gift Box application dashboard and create your Free Shipping Gift:
Click on the New Gift button and select the Free Shipping option.
From there, set up the threshold and shipping price information (the same as in the condition you added to the shipping rate).
Click on the Save button.
P.S.: Remember, you can only create one free shipping gift from the application dashboard.