title: Paypal IPN process more than one custom variable
body: <p>So I've implemented paypal IPN in my site and I'm in the middle of the
  work process. Now I want to use more than 1 custom variable in the pp form
  currently I'm using this one only</p>
  
  <pre><code>&lt;input type="hidden" name="custom" value="&lt;?php echo $user_id; ?&gt;"&gt;
  </code></pre>
  
  <p>So I know the variable with the name 'custom' is allowed. I want to know if
  I can pass more variables so I can filter the payments based on their criterias.
  So if for example shipping is more than $0.00 I set a variable "shipping_cost" like this:</p>
  
  <pre><code>&lt;input type="hidden" name="shipping_cost" value="&lt;?php echo $cost; ?&gt;"&gt;
  </code></pre>
  
  <p>or for other purposes. Is this allowed? Or paypal has an already defined list of allowed
  variables we can use? I really want to solve this problem as there's not always one type
  of payment we can process...
  Thank you guys.</p>