Hi Austin,
This is exactly what you are looking for: https://apps.shopify.com/checkout-boost
To boost your checkout rate, this app allows you to set free gifts, free shipping or discount (of your choice) to customers if they share their cart on Facebook, Twitter,... It will not only encourage them to finish their cart to catch your offer, but it will also help you reach more new customers on social networks.
Give it a try ;)
Helen
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