ajax get checkbox name which is an array

updated
I'm using while loop to display checkbox, the value of the checkbox is the amount. The codes can now display the total value when a checkbox is checked through the ajax function. During form submit, how do i obtain the $ID of checked value? As the name of the checkbox is not an array, i have no idea how to do that other way. Please help :'( Thanks in advance!
PHP:

<table>
<form name="listForm">
<?php
while($row=mysql_fetch_assoc($result)){
$ID = $row['ID'];
$amount = $row['amount'];
?>
<tr>
<td><input type="checkbox" name="payID" data-payment-id="<?php echo $ID;?>" value="<?php echo $amount;?>" onclick="calculate();"></td>

</tr>
<?php
}?>
</table>
</form>

javascript:

function calculate(){
document.listForm.payID.value = '';
var sum = 0;
var count = 0;
for (i=0;i< document.listForm.payID.length ; i++) {
if (document.listForm.payID[i].checked) {
sum = sum + parseInt(document.listForm.payID[i].value);
count = count + 1;
}
}

document.getElementById('payable').innerHTML = "Total record chosen= "+ count +"<br>Total amount= " + sum + "<br>";
}