Pass Javascript variable to hidden field and php form

[microtribe]

I have a Javascript total calculation function in an external .js file called from a php form using onBlur to show the client a running total of the dollar amount of items added: http://www.microtribe.com/dev4Tony/order881x2.php

And I have an external PHP file/script to email the results of the form to me. All works fine but I can't figure out how to grab the total dollar amount ("sub_total") that is calculated by the Javascript and returned to a div tag on my form with this line:
document.getElementById('sub_total').innerHTML = '$ ' + runningTotal.toFixed(2);

Is there a way to have the Javascript send this variable to a hidden field on my form which I can then pass to my php script to email? I am able to get the total into a div tag, but not a hidden field.

Here is the full Javascript Function:

/*Addition Function
*/
var elements = new Array();

function calculatePrice(price,me,id) {
var newValue = 0;
if (me.value == 0) {
document.getElementById('total' + id).innerHTML = '';
} else if (me.value > 0) {
newValue = price * me.value;
document.getElementById('total' + id).innerHTML = newValue.toFixed(2);
}

elements['total' + id] = newValue.toFixed(2);

calculateTotal();

}

function calculateTotal() {
var runningTotal = 0;
for (var strCurrentKey in elements) {
runningTotal += parseFloat(elements[strCurrentKey]);
}

var sub = runningTotal.toFixed(2);
var tax = sub * 0.00;
var total = sub * 1.08375;

document.getElementById('sub_total').innerHTML = '$ ' + runningTotal.toFixed(2);
	document.getElementById('tax').innerHTML = '$ ' + tax.toFixed(2);
	document.getElementById('grand_total').innerHTML = '$ ' + total.toFixed(2);

	document.order_guide.subtotal.value = '$ '+runningTotal.toFixed(2);
	document.getElementById('field_subtotal').value = runningTotal.toFixed(2);
	document.getElementById('field_tax').value = tax.toFixed(2);
	document.getElementById('field_total').value = total.toFixed(2);

}


Here are relevant bits of the php form:
  <form enctype="multipart/form-data" name="order_guide" action="sendorder.php" method="post">
<input name="subtotal" type="hidden" id="subtotal" value="">

And relevant bits of my php script:
	 $sub_total = $_POST['subtotal'];
$msg .= "ORDER SUBTOTAL: $sub_total\n";

Thanks!
Tony