checkbox value form help!

Hello,
I have been at this forever and cannot find any solution
I have 5 checkboxes, the first 3 each have a value of 260, check 2 or more and the total is maxed out at 390. So far so good.
The problem is the 2 other checkboxes. They each have a value of 130. If checked, their values need to be added to the total no matter what. In other words these 2 checkboxes need to be exempt from the max 390 rule.
I just can't get this to work.

code:

<script type="text/javascript">
if (!Array.prototype.indexOf) {
Array.prototype.indexOf = function (needle) {
for (var i = 0; i < this.length; i++) {
if (this[i] === needle) return i;
}
return -1;
};
}
// Reference to our form element, and other things
var form = document.forms["form1"], total = checked = 0,
exempt = ["checkbox_group_4[]", "checkbox_group_5[]"];
// Bind up our calculate method when our form changes
} else if (form.attachEvent) {
form.attachEvent("onchange", calculate);
} else {
form.onchange = calculate;
}
// Define our Calculate function
function calculate () {
// Reset total and checked
total = 0, checked = 0;
// Cycle over each item in our form
for (var i = 0; i < form.length; i++) {
// Handle only those that are checked
if (form[i].checked) {
// Add their value to the total value
total += parseInt(form[i].value, 10);
// If they aren't extempt from flatrate
if (exempt.indexOf(form[i].name) < 0) {
// Increment the number of eligible items
checked += 1;

}

}
}
// If more than one eligible item is checked
if (checked > 1) {
total = 390;

// Else if only one eligible item is checked
}
else if (checked > 0) {
total += 0;
}

// Show the user the estimated value
form["total"].value = total;
}

};
</script>

Form:

<form id="contactForm" name="form1" action="send-mail.php" method="POST"><fieldset>

<input type="checkbox" name="checkbox_group_1[]" onclick="calculate()" value="260" class=" " title="Cochez ici" />
<label>Mardi 19h à 21h30</label> <br/>

<input type="checkbox" name="checkbox_group_2[]" onclick="calculate()" value="260" class=" " title="Cochez ici" />
<label>Vendredi 19h à 21h30</label>

<input type="checkbox" name="checkbox_group_3[]" onclick="calculate()" value="260" class=" " title="Cochez ici" />
<label>Vendredi 19h à 21h30</label>

<input type="checkbox" name="checkbox_group_4[]" onclick="calculate()" value="260" class=" " title="Cochez ici" />
<label>Mardi 19h à 21h30</label> <br/>

<input type="checkbox" name="checkbox_group_5[]" onclick="calculate()" value="260" class=" " title="Cochez ici" />
<label>Vendredi 19h à 21h30</label>

<input type="text" size="2" name="total" value="0"/>

</fieldset>
</form>