<label class="radio">
  <input type="radio" id="frete_radio" name="frete_radio" value="1">
  <span>a</span>
</label>
<label class="radio">
  <input type="radio" id="frete_radio" name="frete_radio" value="2">
  <span>b</span>
</label>

<button type="submit" id="btn-finalizar" disabled="disabled">

<script type="text/javascript">
    $(document).ready(function() {
        $("input[type=radio]").bind("click", function(){
            if($("input[type=radio]:checked").val() == "frete_radio") {
                $("#btn-finalizar").attr({disabled: true});
            } else {
                $("#btn-finalizar").attr({disabled: false});
            }
        });
    });
</script>