you could add a form input to the cehckout page:
<input name="promoter" type="text" id="promoter" value="
" />
and add a hidden form element of the same name on the confirm page to capture the value passed from checkout:
<input name="promoter" type="hidden" id="promoter" value="<?php echo((isset($_POST["promoter"]))?$_POST["promoter"]:"") ?>" />
also on the confirm page, capture the promoter into a session variabl.
Go to the server behaviors panel and click the plus button. Select eCart -> General -> Set Session Value.
set the trigger to any form post.
Set the name to promoter
click the lightning bolt next to value and select the promoter form element.
finally, you will want to add a promoter column to the orders table.
then edit the eCart Store Order Summary on the confirm page, bind the promoter table column to the promoter session variable.