Paypal Subscription Modification

Question

Hi, I was hoping someone could help regarding modifying a subscription price.

I have a social networking site In which you originally could subscribe to a standard or premium account which varied in what applications where active in your account.

My client now whats to scrap the 2 subscriptions and be able to give a choice of what applications the user wants to use and charge a subscription cost for the total cost for the applications specified.

I have a 'bolt-ons' page in which the user clicks a checkbox associated with each application, which then processes the payment and the IPN makes modifications to their account which works fine.

But when a user decides they want more or less applications and fills out the form again, I can't figure out how to modify their payment.

when I add the modify variable to the html form and continue the payment, it brings me to a radio form with other subscriptions from different projects.

I simply want to modify the total outgoings of the users payments. It is the user the process's the form, there is nothing illegitimate going off, just simply looking for the user to be able to change the subscription rate which depends on what applications they want active.

here's my html form:

<form action="https://www.paypal.com/cgi-bin/webscr" method="post" 

style="margin:20px;">
<input type="hidden" name="cmd" value="_xclick-subscriptions">
<input type="hidden" name="business" value="******************************" />
<input type="hidden" name="item_name" value="************************ Application Bolt-on's" />
<input type="hidden" name="return" value="http://www.***************.co.uk/?navigation=home"&gt;
<input type="hidden" name="cancel_return" value="http://www.***************.co.uk/?navigation=home"&gt;
<input type="hidden" name="item_number" value="1" />
<input type="hidden" name="currency_code" value="GBP" />
<?php
if( isset( $post['mod'] ) ) {
    echo "<input type=\"hidden\" name=\"modify\" value=\"2\" />";   
    echo "<input type=\"hidden\" name=\"a3\" value=\"00.02\" />";
}else{
    echo "<input type=\"hidden\" name=\"a3\" value=\"00.01\" />";
}
?>
<input type="hidden" name="p3" value="1" />
<input type="hidden" name="t3" value="M" />
<input type="hidden" name="src" value="1" />
<input type="hidden" name="sra" value="1" />
<input type="hidden" name="sra" value="1" />
<input type="hidden" name="no_note" value="1" />
<input type="hidden" name="custom" value="<?php echo $custom_str; ?>" />
<input type="image" src="https://www.paypal.com/en_US/GB/i/btn/btn_subscribeCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online.">
<img alt="" border="0" src="https://www.paypal.com/en_GB/i/scr/pixel.gif" width="1" height="1">
</form>

I have read many articles and non touch on this subject in a sense of whether it can or cannot be done so please don't send me links the the paypal site unless it is the exact answer which I may have missed.

Any help is much appreciated,

Regards,

Phil