title: Stripe - Add new card to existing customer
body: <p>I have a need to add a card to a pre-existing customer. Here's what I did:</p>
  
  <h1>1. obtain token from user submission</h1>
  
  <pre><code>card_token = request.POST('stripeToken')
  </code></pre>
  
  <h1>2. retrieve a customer</h1>
  
  <pre><code>customer =  stripe.Customer.retrieve('cus_xxxxxxxxxx')
  </code></pre>
  
  <h1>3. add card to this customer</h1>
  
  <pre><code>customer.Cards.create(card=card_token)
  </code></pre>
  
  <p>It is <strong># 3</strong> that I'm having trouble because it looks like the customer doesn't have method Cards, but I've seen people done it elsewhere. </p>
  
  <p>How should I achieve this?</p>