title: How to create an invoice for magento downloadable product?
body: <p>I have an problem with invoice generating for my downloadable product.</p>
  
  <p>I tried to create an invoice. But it shows like this error message <strong>Cannot create an invoice without products</strong>.</p>
  
  <p>Can any one help to solve this issue.</p>
  
  <p>Thanks</p>