Dynamically total textbox and checkbox

[bird]

Greetings,

I'm trying to calculate the total on an php page. I'm able to get the
checkbox portion to work properly but I'm having trouble with the
textbox "txtshipping". The default is 7.95 but if changes I need to
change the total.

How can I make the totalbox the default of the textbox of
"txtshipping"?

I struggle with javascript so any help would be greatly appreciate.

Thanks in advance.
Bird

<script type="text/javascript">

function gettotal(){

var tmp = 0;
var nBox = document.getElementsByName('chkboxpackage[]');
for(i=1; i<nBox.length+1; i++)
{
if (nBox[i-1].checked)
{
tmp += parseFloat(document.forms[0]['hdnPackage'+i].value);

}
}

var nBox2 = document.getElementsByName('chkboxitem[]');
for(i=1; i<nBox2.length+1; i++)
{
if (nBox2[i-1].checked)
{
tmp += parseFloat(document.forms[0]['hdnItem'+i].value);

}
}

tmp += parseFloat(document.forms[0]['txtshipping'].value);


document.forms[0].totalbox.value = tmp.toFixed(2);
}

</script>

<input type="checkbox" name="chkboxpackage[]" value="1"
onClick="gettotal()">
<input type="hidden" name="hdnPackage1"
value="69.95" >

<input type="checkbox" name="chkboxpackage[]" value="2"
onClick="gettotal()">
<input type="hidden" name="hdnPackage2"
value="49.95" >
<input type="checkbox" name="chkboxpackage[]" value="3"
onClick="gettotal()">
<input type="hidden" name="hdnPackage3"
value="495.00" >
<input type="checkbox" name="chkboxitem[]" value="1"
onClick="gettotal()">
<input type="hidden" name="hdnItem1"
value="89.95" >
<input type="checkbox" name="chkboxitem[]" value="2"
onClick="gettotal()">
<input type="hidden" name="hdnItem2"
value="99.95" >
<input type="checkbox" name="chkboxitem[]" value="3"
onClick="gettotal()">
<input type="hidden" name="hdnItem3"
value="149.95" >


<input name="txtShipping" type="text" size="15"
onChange="gettotal()"></td>
<input type="text" name="totalbox"
value="0.00" readonly/>