There is a lot of ways to achieve this. In regards to the way you are asking, with a hidden form element.
create this form element inside your form:
<input type="hidden" name="total" value="">
So your form like this:
<form id="sampleForm" name="sampleForm" method="post" action="phpscript.php">
<input type="hidden" name="total" id="total" value="">
<a href="#" onclick="setValue();">Click to submit</a>
</form>
Then your javascript something like this:
<script>
function setValue(){
document.sampleForm.total.value = 100;
document.forms["sampleForm"].submit();
}
</script>