Mapbox comes with a free tier. Please refer to their pricing page to see if you'll fall under free usage of the API key - https://www.mapbox.com/pricing#maps
Summary
In order to use some of the features on our theme like the store locator you’ll need to create and update a Mapbox API key.
How to
Generate an API key
Visit Mapbox
Sign up for a new developer account
Create a new API key