Americart Support
posted this on September 05, 2012 18:30
This article will illustrate how to create a Free Shipping Custom Shipping Method applied to orders over $50.


Give the custom shipping method a name and a description (optional). This name is what will show to your customers as the shipping option on the storefront. When you are ready to use this method make sure the Active checkbox is checked.
In order to work the shipping method must have rules. The rule should read out like a normal sentence. In this case we want free shipping on orders over $50. If you want the shipping apply to ANY order make the range 0 to 999999 The rule requires a lower and an upper value so unless there is an alternate rule, make the upper value very high. The most important fields are the ones that say order is between $ and $, and shipping charge is $.

Once you add the rule you MUST click the yellow plus sign to the left to add the rule to the shipping method.

Unless you want different charges when other conditions are met, the one rule is all you will need for the method.
Click save and let your free shipping method go to use
Note: In order for the shipping drop down to display on the shopping cart the option must be enabled in your theme.
To check if this is enabled go to Themes > [Active] > Catalog Pages > Shopping Cart, and make sure the following options are similar.

Comments
Having the ability and configuration flexibility is awesome. The question is how does one arrive at the threshold amount?
That's a good question. However as you've found there is no simple answer or a formula that you can follow all the time. It completely depends on your customer base, your profit margins, average shipping cost, etc. If your average order amount is small and the profit doesn't cover the shipping cost... you're gonna have a bad time.
Here's an article I found discussing the topic:
http://webmarketingtoday.com/articles/bulger-free-shipping/