Zencart Tutorial – Free Shipping for Bigger Orders
Many businesses offer free shipping for orders, worth above a certain amount. You can do that in ZenCart as well. Point your cursor to the Modules menu, and select the Order total option. From the list, click the Shipping module and then the Edit button.

Set the minimum amount for free shipping and click Update at the bottom for your changes to be applied.

Source: http://www.siteground.com/tutorials/zencart/
This tutorial is provided free of charge and without any warranty of any kind.