Hi Brian and Wei Yang,
You may want to take a look at this app: https://apps.shopify.com/checkout-boost
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.
Free ship / Free gift / Discount can be set for many purposes:
Hope it helps :)
|an hour ago|