If I have a store that offers flat rate shipping of $10 and free shipping for orders over $100 and a customer orders over $100 it doesn’t make much sense for them to have to see both free shipping and flat rate shipping on the checkout page. Why would a user even choose flat rate shipping?

How can I use a filter to detect if free shipping is available, filter the flat rate shipping label so it says something like flat rate shipping (free), and then disable the free shipping method.

The end result is that if a user orders less than $100 they see flat rate shipping $10 and if they order over $100 they see flat rate shipping (free!).