Problem to enrol the course when when the cost is zero.

Hi,
Let’s see if everyone can help me.
I’m using in my web site:
-Joomla 1.5.23;
-Joomdle 0.54;
- Moodle 1.9.13;
- VirtueMart 1.1.9 (using the PayPal pay method) ;
Everything is working very well, but when I offer a free course (e.g. U$=0) in this case, The user is not submitted to PayPal web site and consequently, we have no feedback from PayPal and the user is not enrolled by Joomdle. So I would like to know if someone could help me to solve this problem. Thank you very much!
The code is:

<?php
$db1 = new ps_DB();
$q = "SELECT country_2_code FROM #__vm_country WHERE country_3_code='".$user->country."' ORDER BY country_2_code ASC";
$db1->query($q);
$url = " www.paypal.com/cgi-bin/webscr k";
$tax_total = $db->f("order_tax") + $db->f("order_shipping_tax");
$discount_total = $db->f("coupon_discount") + $db->f("order_discount");
$post_variables = Array(
"cmd" => "_ext-enter",
"redirect_cmd" => "_xclick",
"upload" => "1",
"business" => PAYPAL_EMAIL,
"receiver_email" => PAYPAL_EMAIL,
"item_name" => $VM_LANG->_('PHPSHOP_ORDER_PRINT_PO_NUMBER').": ". $db->f("order_id"),
"order_id" => $db->f("order_id"),
"invoice" => $db->f("order_number"),
"amount" => round( $db->f("order_subtotal")+$tax_total-$discount_total, 2),
"shipping" => sprintf("%.2f", $db->f("order_shipping")),
"currency_code" => $_SESSION,
"address_override" => "1",
"first_name" => $dbbt->f('first_name'),
"last_name" => $dbbt->f('last_name'),
"address1" => $dbbt->f('address_1'),
"address2" => $dbbt->f('address_2'),
"zip" => $dbbt->f('zip'),
"city" => $dbbt->f('city'),
"state" => $dbbt->f('state'),
"country" => $db1->f('country_2_code'),
"email" => $dbbt->f('user_email'),
"night_phone_b" => $dbbt->f('phone_1'),
"cpp_header_image" => $vendor_image_url,
"return" => SECUREURL ."index.php?option=com_virtuemart&page=checkout.result&order_id=".$db->f("order_id"),
"notify_url" => SECUREURL ."administrator/components/com_joomdle/paypal_notify.php",
"cancel_return" => SECUREURL ."index.php",
"undefined_quantity" => "0",
"test_ipn" => PAYPAL_DEBUG,
"pal" => "NRUBJXESJTY24",
"no_shipping" => "1",
"no_note" => "1"
);
if( $page == "checkout.thankyou" ) {
$query_string = "?";
foreach( $post_variables as $name => $value ) {
$query_string .= $name. "=" . urlencode($value) ."&";
}
vmRedirect( $url . $query_string );
} else {
echo '<form action="'.$url.'" method="post" target="_blank">';
echo '<input type="image" name="submit" src=" www.paypal.com/en_US/i/btn/x-click-but6.gif k" alt="Click to pay with PayPal - it is fast, free and secure!" />';
foreach( $post_variables as $name => $value ) {
echo '<input type="hidden" name="'.$name.'" value="'.htmlspecialchars($value).'" />';
}
echo '</form>';
}
?>

Hi Antonio,
Thank you very much for you quickly feedback.
Have you already did something like it? Do you have any idea how I can do it?!
If you have step by step I will be very grateful, because I don’t have many experience, I am beginning and learning in my web site.

I was wondering another way to try to solve it, but I also not sure how I can do it. When the buy process is concluded in the VM a email is sent to the customer, so do you know if it is possible to add in this email a enroll joomdle link of the course?

I don`t know which is easer?
Thanks for your help

Olá Antonio,
PS: observei em um outro tópico que você compreende português... acho que consigo te explicar melhor assim...
Como não é mais possível utilizar o paypal para vender para usuário não cadastrados no paypal, estou partindo para uma outra solução que é o pagseguro, também bastante utilizado aqui no Brasil. Mas ai estou batendo no mesmo problema anterior, que é de quando tenho cursos com custo zero e o VM reconhece e confirma a aquisição do curso, porém não é notificado o Joomdle e ai a matricula não é realizada....
Não existe ou seria muito difícil e caro gerar um plugin que quando confirmado a compra no VM o VM gerasse uma notificação para o Joomdle, e ai sim realizando a matricula no moodle. Se tiver idéia de custos para essa personalização me avisa, acho que seria bastante interresante e deixaria o Joomdle desvinculado da forma de pagamento e sim ao status do VM.
Novamente obrigado Antonio
Ital