There is a powerful feature within the Marketing Hub > Conversion Lab > Signify, one of the signify types is called Dynamic Free Shipping. This allows the user to configure, customize and display a message in the cart, and/or in the entire store, with the offer of FREE shipping if the cart value reaches a certain value. 

For example, FREE Shipping for orders over $75. The notification is dynamic in that it adjusts with the amount that is in your cart. 

Check out the video below to see a demonstration of configuring the dynamic free shipping Signify type:

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