This is being caused by the server side validation of the zip code.
find the following line:
$WAFV_Errors .= WAValidateZC(((isset($_POST["UserZip"]))?$_POST["UserZip"]:"") . "",true,true,true,true,true,18);
and comment it:
//$WAFV_Errors .= WAValidateZC(((isset($_POST["UserZip"]))?$_POST["UserZip"]:"") . "",true,true,true,true,true,18);