You can create a discount code that automatically gives free delivery for your customers.
Go to Products > Discounts and click
Create Discount
Choose "Free delivery"
Set your discount code parameters:
Discount code: The code that will be shared to your buyer
Minimum purchase requirement: Not applicable / minimum purchase amount / minimum quantity of items
Maximum discount usage: Not applicable / total usage / usage per customer
Active dates: the starting and end date (if applicable)
Click "Save".
Once created, share your discount code with your customers and track its usage from the Discounts section.