The Check box works, it just does not auto fill the shipping fields...
<input type="checkbox" name="eC_SameAs" id="eC_SameAs" value="1" onclick="eCart_copyBillingToShipping(this);">
Here's a sample of the first field for first name in shipping:
<input type="text" name="shipping_firstname" id="shipping_firstname" value="">
as opposed to the billing field:
<input type="text" name="firstname" id="firstname" value="">