title: How to manage IPN url and notify_url
body: <p>The account backend give you the ability to choose a url for IPN calls. That url is unique.
  On the other hand, the html form has a notify_url parameter (i always used that one to structure my flows dynamics) and it can change based on some variables.</p>
  
  <p>The question is: which is the bound between the two parameters URLs? If i set the notify url in the form, i should follow some good habits in choosing the IPN url in PP account backend? And vice-versa?</p>
  
  <p>Thanks!</p>