First of all, it is necessary to check if the delivery method that you are going to rewrite is turned on at Magento Admin Panel → Stores → Configuration → Sales → Shipping Methods:
Then, create a new shipping rule at Magento Admin Panel → Sales → Shipping Rules → Add Rule. Select the required method from the Carriers and Methods dropdown and set the cart subtotal condition in Conditions tab:
In Rates tab, choose Replace action from the Calculation dropdown, make sure the Base Rate for the Order value equals 0, and save the rule:
With that, the original delivery charges are applied when the subtotal is less than $100: