I dug around the Intertubes and found this link where there's a great example of a semi-complex html form, with javascript support, that sets up a multiple-control payment form. Unfortunately, the formatting on that page is horrible, making the javascript useless. Here is my modified version of it, formatted a little better for posterity sake:
It's a form with two user elements, a select list (dropdown box), and a checkbox that adds an annual fee if appropriate. The form submits the calculated total to the PayPal payment form.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<title>
Powder Mountain Tours
</title><!-- Start of Script -->
<script type="text/javascript">
<!-- //
function Dollar (val) { // force to valid dollar amount
var str,pos,rnd=0;
if (val < .995) rnd = 1; // for old Netscape browsers
str = escape (val*1.0 + 0.005001 + rnd); // float, round, escape
pos = str.indexOf (".");
if (pos > 0) str = str.substring (rnd, pos + 3);
return str;
}
var amt,des,obj,val,op1a,op1b,op2a,op2b,itmn;
function ChkTok (obj1) {
var j,tok,ary=new Array (); // where we parse
ary = val.split (" "); // break apart
for (j=0; j<ary.length; j++) { // look at all items
// first we do single character tokens...
if (ary[j].length < 2) continue;
tok = ary[j].substring (0,1); // first character
val = ary[j].substring (1); // get data
if (tok == "@") amt = val * 1.0;
if (tok == "+") amt = amt + val*1.0;
if (tok == "%") amt = amt + (amt * val/100.0);
if (tok == "-") amt = amt - val*1.0;
if (tok == "#") { // record item number
if (obj1.item_number)
obj1.item_number.value = val;
ary[j] = ""; // zap this array element
}
// Now we do 3-character tokens...
if (ary[j].length < 4) continue;
tok = ary[j].substring (0,3); // first 3 chars
val = ary[j].substring (3); // get data
if (tok == "s1=") { // value for shipping
if (obj1.shipping)
obj1.shipping.value = val;
ary[j] = ""; // clear it out
}
if (tok == "s2=") { // value for shipping2
if (obj1.shipping2)
obj1.shipping2.value = val;
ary[j] = ""; // clear it out
}
}
val = ary.join (" "); // rebuild val with what's left
}
function StorVal () {
var tag;
tag = obj.name.substring (obj.name.length-2); // get flag
if (tag == "1a") op1a = op1a + " " + val;
else if (tag == "1b") op1b = op1b + " " + val;
else if (tag == "2a") op2a = op2a + " " + val;
else if (tag == "2b") op2b = op2b + " " + val;
else if (tag == "3i") itmn = itmn + " " + val;
else if (des.length == 0) des = val;
else des = des + ", " + val;
}
function ReadForm (obj1, tst) { // Read the user form
var i,j,pos; amt=0;des="";op1a="";op1b="";op2a="";op2b="";itmn="";
if (obj1.baseamt) amt = obj1.baseamt.value*1.0; // base amount
if (obj1.basedes) des = obj1.basedes.value; // base description
if (obj1.baseon0) op1a = obj1.baseon0.value; // base options
if (obj1.baseos0) op1b = obj1.baseos0.value;
if (obj1.baseon1) op2a = obj1.baseon1.value;
if (obj1.baseos1) op2b = obj1.baseos1.value;
if (obj1.baseitn) itmn = obj1.baseitn.value;
for (i=0; i<obj1.length; i++) { // run entire form
obj = obj1.elements[i]; // a form element
if (obj.type == "select-one") { // just selects
if (obj.name == "quantity" || obj.name == "amount") continue;
pos = obj.selectedIndex; // which option selected
val = obj.options[pos].value; // selected value
ChkTok (obj1); // check for any specials
if (obj.name == "on0" || obj.name == "os0" || obj.name == "on1" || obj.name == "os1") continue;
StorVal ();
}
else if (obj.type == "checkbox" || // just get checkboxex
obj.type == "radio") { // and radios
if (obj.checked) {
val = obj.value; // the value of the selection
ChkTok (obj1);
StorVal ();
}
} else if (obj.type == "select-multiple") { //one or more
for (j=0; j<obj.options.length; j++) { // run all options
if (obj.options[j].selected) {
val = obj.options[j].value; // selected value (default)
ChkTok (obj1);
StorVal ();
}
}
}
else if ((obj.type == "text" || // just read text,
obj.type == "textarea") && obj.name != "tot" && // but not from here
obj.name != "quantity") {
val = obj.value; // get the data
if (val == "" && tst) { // force an entry
alert ("Enter data for " + obj.name);
return false;
}
StorVal ();
}
}
// Now summarize stuff we just processed, above
if (op1a.length > 0) obj1.on0.value = op1a;
if (op1b.length > 0) obj1.os0.value = op1b;
if (op2a.length > 0) obj1.on1.value = op2a;
if (op2b.length > 0) obj1.os1.value = op2b;
if (itmn.length > 0) obj1.item_number.value = itmn;
obj1.item_name.value = des;
obj1.amount.value = Dollar (amt);
if (obj1.tot) obj1.tot.value = "$" + Dollar (amt);
} //-->
</script><!-- End of Script -->
</head>
<body>
<h1>Winter 2013 Gymnastics</h1>
<h3>Registration payment form</h3>
<form action="https://www.paypal.com/cgi-bin/webscr" method="post"
onsubmit="this.target = 'paypal'; return ReadForm(this, true);">
<p>
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="myEmail@myUrl.com">
<!--your email address goes here -->
<input type="hidden" name="item_name" value=""> <input type="hidden" name="amount" value="">
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="cancel_return" value="http://myUrl.com/content/payment-cancelled">
<input type="hidden" name="return" value="http://myUrl.com/content/payment-confirmed">
<input type="hidden" name="baseamt" value="00">
<input type="hidden" name="basedes" value="Gymnastics Class">
<input type="hidden" name="baseon0" value="">
<input type="hidden" name="baseos0" value="">
<input type="hidden" name="baseon1" value="">
<input type="hidden" name="baseos1" value="">
<input type="hidden" name="baseitn" value="">
<input type="hidden" name="on0" value="">
<input type="hidden" name="os0" value="">
<input type="hidden" name="on1" value="">
<input type="hidden" name="os1" value="">
<input type="hidden" name="item_number" value="">
<b>Select Your Class: </b><br/>
<select name="package" onchange=
"ReadForm (this.form, false);">
<option value="Tue 5:00 @48.00" selected>
Tuesday, 5:00-5:40p.m. Ages 3 to 4 1/2 - $48.00
</option>
<option value="Tue 5:40 @52.00">
Tuesday, 5:40-6:35p.m. Ages 4 1/2 to 7 - $52.00
</option>
<option value="Tue 6:35 @64.00">
Tuesday, 6:35-8:00 p.m. Ages 7 & up - $64.00
</option>
<option value="Thu 5:00 @52.00">
Thursday, 5:00-5:50 p.m. Ages 5 to 6 - $52.00
</option>
<option value="Thu 5:50 @52.00">
Thursday, 5:50-6:45 p.m. Ages 6 to 8 - $52.00
</option>
<option value="Thu 6:30 @64.00">
Thursday, 6:30-8:05 p.m. Ages 9 & up - $64.00
</option>
</select><br>
<br>
<b>Annual registration fee:</b> <br>
(check this box if this gymnast didn't attend the fall 2012 session)
<br>
<input type="checkbox" onclick="ReadForm (this.form, false);" value=
"Annual Fee +10.00"> Annual Registration Fee $10.00<br>
<br>
<br>
<input type="image" src=
"https://www.paypal.com/en_US/i/btn/btn_paynowCC_LG.gif" name="submit"
alt="cart add"> Total Cost > <input class="nbor" type="text" name=
"tot" size="8" value="$48.00"> <input type="button" value="Reset FORM"
onclick="this.form.reset();">
</p>
</form>
</body>
</html>