In short: Add a Global Condition Cart Subtotal Greater than or equal to 100, then add a 20% Product Discount and a Shipping Discount with Free Shipping. Both apply once the cart reaches $100.
Why one code: Shopify's own discount codes each hold one kind of discount, so 20% off and free shipping would be two codes, or a code plus free shipping you give every customer. OneCode: Stackable Discounts ties both to one code.
Steps
Click Create discount and enter a code, for example
SPEND100.In Global Conditions, click Add Condition. Set Field to Cart Subtotal, Operator to Greater than or equal to and Value to
100.Click Add Discount Type and enter
20in Percentage. Keep Target as All Products.Click Add Discount Type, set Discount Type to Shipping Discount and keep Free Shipping.
Click Save discount.
What the customer gets
$120 cart: 20% off every item ($24 saved) and free shipping.
$80 cart: no discount and no free shipping, because the global condition isn't met.
Variations
Free shipping from $75, 20% off from $100: put no global condition. Give the product discount a Per-Discount Condition Cart Subtotal at least
100, and the shipping discount Cart Subtotal at least75.Only the cheapest rate free: set Rate Selection to Cheapest Rate.
Free shipping only in your country: add the condition Shipping Country Equals
USto the shipping discount.Lower free shipping minimum for VIPs: put the $100 on each discount type instead of in Global Conditions. Then add a second free shipping discount with Cart Subtotal at least
50and Customer Tag Is one ofVIP.Half-price express too: with Cheapest Rate on the free shipping, add a shipping discount of
50Percentage with Rate Selection set to Select Rates and your express rate's exact checkout name added, for exampleExpress Shipping.
Good to know
The $100 is checked before the 20% comes off. A $110 cart still qualifies even though it's $88 after the discount.
Free gift over $100: add a product discount of
100% targeting the gift product, with a Quantity Limit of1. The customer still adds the gift to their cart themselves, and the gift's price counts toward the $100, so set the threshold to your minimum plus the gift's price (for example120for a $20 gift).