Great article from Mark Hayes at Shopify about the 4 Magic words that increase sales.
As an example, a retailer in the Northwestern US offers free shipping on some orders. The retailer first offered free shipping for orders of $50 or more. On average shoppers spent a little more than $50 on the site.
In November 2012, the policy changed, so that free shipping required a minimum $75 purchase. The retailer’s average order rose to nearly $80. People spent more just to get something ‘free’ – so consider offering free shipping after a certain purchase threshold on your ecommerce store.