Custom Shipping Prices By Shipping Method - Free Shipping For Ground
How can I offer free shipping?
The answer to this question really depends on which shipping cost calculator is being used.
In most cases you can navigate to Settings / Shipping / Next (to page 2) / Next (to page 3). On this screen uncheck RTR (real-time rates) for any of the methods that you would like to be $0 and click next. On page 4 enter the price you would like to charge ($0 in this case) for the methods that where RTR was unchecked.