Many websites offer free shipping in the USA. The default shipping module for zencart does not allow a store owner to easily handle it. In the configuration of the shipping module, you set the desired country (USA) to display free shipping to, and any of the other options that are available in the standard freeshipper module included with zencart. The zip file contains just 2 files, the shipping total, and a english language define. Install instructions are included in the download. Very easy and fast to install.
You may download the module form bitbucket.org, or even help contribute to it.