Have a look in your WA_eCart folder in your site and see if it contains a folder called scripts which contains a file called copyShipping.js
If it does you just need to link to it in the head of your page and put a checkbox like so:
<label>
<input type="checkbox" name="eC_SameAs" id="eC_SameAs" value="1" onclick="eCart_copyBillingToShipping(this);" /> Same as billing address</label>
on your page in the appropriate place.