<form name action="rep_nar.jsp" method="get">
<input name="it_snds" id="it_snds" type="hidden" value="0.00"/>
</form>
........
<c:set var="nit_snds" value="${Row.snds}"/>
    <script language="javascript">
      elem=document.getElementById("it_snds");
      elem.value=nit_snds // как правильно передать?
      alert(elem)
    </script>
<c:set var="nit_snds" value="${it_snds}"/>
........

<c:set var="nit_snds" value="0.00"/>
<c:set var="nit_snds" value="${nit_snds+39347629.66}"/>

<c:set var="nit_snds" value="${nit_snds+39347629.66}"/>