How to get the value of paypal options in IPN

My paypal Buy Now buttons include a selection where the customer can select from a drop down blox with the item and price.

My problem now is that how can I identify which of the option was the customer selected in my ipn script.

here part of the paypal button

input type="hidden" name="on0" value="Credits">Buy Additional Credits <br /> 

<select name="os0"> 
<option value="3 Credit">$24 - 3 Credit</option> 
<option value="5 Credit">$40 - 5 Credit</option> 
<option value="7 Credit">$56 - 7 Credit</option> 
<option value="10 Credit">$80 - 10 Credit</option> 
</select> 
<br /> 
<!-- Specify the price that PayPal uses for each option. --> 

<input type="hidden" name="option_index" value="0"> 
<input type="hidden" name="option_select0" value="3 Credit"> 
<input type="hidden" name="option_amount0" value="1.00"> 
<input type="hidden" name="option_select1" value="5 Credit"> 
<input type="hidden" name="option_amount1" value="2.00"> 
<input type="hidden" name="option_select2" value="7 Credit"> 
<input type="hidden" name="option_amount2" value="3.00"> 
<input type="hidden" name="option_select3" value="10 Credit"> 
<input type="hidden" name="option_amount3" value="4.00">

Code (markup):

I am thinking of doing this but not sure if this will work

$price1 = $_POST[option_amount0];
$price2 = $_POST[option_amount1];
$price3 = $_POST[option_amount2];
$price4 = $_POST[option_amount3];

Code (markup):

 

I think what you're doing will work, however i've always used single quotes, like so:

$price1 = $_POST['option_amount0'];

Code (markup):