I am beginning to create a form to let customers get quotes depending on the options they select. I was doing fine getting the total to work until I added the check box. Why will it not work now?

PHP Code:
<head>
<
meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<
title>Quote</title>
<
script type="text/javascript">
function 
calc(){
frm document.form1;
n1 frm.select_1.options[frm.select_1.selectedIndex].value;
n2 frm.select_2.options[frm.select_2.selectedIndex].value;
n3 frm.envelope.options[frm.envelope.selectedIndex].value;
if (
printed.checked) { n4 = (printed.value); }
res parseFloat(n1) + parseFloat(n2) + parseFloat(n3) + parseFloat(n4);
frm.result.value res;
}
</script>
</head>
<body>
<form name="form1" id="form1" method="post" action="">
    <p>What style invitation do you want :
        <select name="select_1">
            <option selected="selected">INVITATION SELECTION</option>
            <option value=".50">Standard invitation ($0.50 each)</option>
            <option value="1.20">Metallic invitation ($1.20 each)</option>
            <option value="1.50">Textured Metallic invitation ($1.50 each)</option>
            <option value="2.00">Fabric invitation ($2.00 each)</option>
        </select>
        <br />
        Do you want an envelope liner:
        <select name="select_2">
            <option selected="selected">LINER SELECTION</option>
            <option value="1.00">Standard color liner ($1.00each )</option>
            <option value="2.00">Metallic liner ($2.00 each)</option>
        </select>
        <br />
        What type of Envelope
        <select name="envelope" >
            <option selected="selected">ENVELOPE SELECTION</option>
            <option value=".42">envelope in E1 color ($0.42 each)</option>
            <option value=".50">envelope in E2 color ($0.50 each)</option>
            <option value=".60">envelope in E3 color ($0.60 each)</option>
        </select>
        <br />
        Do you want the envelope printed
        <input name="printed" type="checkbox" id="printed" value=".50" />
    </p>
    <p>
        <input type="button" name="Button" value="Calculate" onclick="calc()" />
The total cost per invitation is 
<input name="result" type="text" id="result" />
    </p>
</form>
</body>
</html>