One more variant is offering to buy products for the fixed amount of money and get free shipping. It's good if you wish to sell more or upsell.
Shopify app to use:
Free Shipping Bar app.
How it works: Shoppers get to your store and see a bar with a message, for instance, "Free shipping over $100". When a product is added to the cart, the message on the bar is changed into "Only $59 away from free shipping".
The app makes a dynamic bar where the text is changed online according to taken actions. In case the trigger on the bar affects a consumer, the last message will say "Congratulations! You've got free shipping".
The individual message is addressed to the concrete person that's why it's forceful.