Distance Based Shipping Integration Index
This service leverages the Google Distance Matrix API. To use it you need to sign up for an API key. Google offer various pricing plans described here. The good news is that they allow you to query their service 2500 times a day without being charged. Once you go past that limit you should be doing enough business to not be overly worried about paying for the service
As off December 2016 you get 2500 requests a day, each additional 1000 requests cost $0.50 USD.