I have the following code on my webpage

HTML Code:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="hosted_button_id" value="3DDXJB52EYN2N">
<table>
<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>
</table>
<input type="hidden" name="currency_code" value="GBP">
<input type="hidden" name="custom" value="<?php echo (''.$id.''); ?>">
<input type="image" src="https://www.paypalobjects.com/en_US/GB/i/btn/btn_buynowCC_LG.gif" border="0" name="submit" alt="PayPal  The safer, easier way to pay online.">
<img alt="" border="0" src="https://www.paypalobjects.com/en_GB/i/scr/pixel.gif" width="1" height="1">
</form>
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