You can create a shipping condition in the “Shipping and Delivery” section of the shipping settings.
Head to the shipping and delivery section of shipping settings in your Shopify admin. Next to the shipping profile, you can add free shipping. Under the shipping profile, click on manage rates and add free shipping rates. Under this section, you can create 2 shipping conditions;
1. Free shipping, if the total cart price is $21
2. $5 shipping charge, if the total cart price is less than $21.