Paypal IPN

I have the following code on my webpage

<form action="" method="post" target="_top">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="hosted_button_id" value="3DDXJB52EYN2N">
<tr><td><input type="hidden" name="on0" value="Credit Amount">Credit Amount</td></tr><tr><td><select name="os0">
	<option value="5">5 £0.60 GBP</option>
	<option value="10">10 £1.20 GBP</option>
	<option value="15">15 £1.80 GBP</option>
	<option value="20">20 £2.40 GBP</option>
</select> </td></tr>
<input type="hidden" name="currency_code" value="GBP">
<input type="hidden" name="custom" value="<?php echo (''.$id.''); ?>">
<input type="image" src="" border="0" name="submit" alt="PayPal – The safer, easier way to pay online.">
<img alt="" border="0" src="" width="1" height="1">

On the paypal IPN what post value is going to send the option value amount?
I have the custom field correct but what about the option value