title: Paypal subscription button
body: <p><strong>Background</strong>
  I'm building a website where I need to use some sort of recurring billing payment method. My client does not have Paypal's Website Payments Pro so he decided to use a simple <code>Subscribe</code> button.</p>
  
  <p><strong>Problem</strong>
  This is all nice and easy to use but <em>how would I get a notification from Paypal when such a payment has been made</em>? I had some experience with Paypal before and there was a field <code>return_url</code> that would be accessed by Paypal to notify the website but checking the variables on their site this field does not exist anymore or perhaps not available with this type of button.</p>