title: Cart add_fee() is not working in checkout page
body: <p>In WooCommerce, I would like to make a discount without using coupons, and the discount calculation will be based on product price, with something like "take 3 products for the price of 2.</p>
  
  <p>In <strong><code>function.php</code></strong> of my active theme I am using this code:</p>
  
  <pre><code>function promo () {
      if (is_cart()) {
          $woocommerce-&gt;cart-&gt;add_fee( __('des', 'woocommerce'), -50.00`enter code here`, true, '');
      }
   }
  add_action ('woocommerce_cart_calculate_fees', 'promo');
  </code></pre>
  
  <p>My problem: This code doesn't work on checkout page.<br>
  If I force in review-order the discount appears, but the total value don't change. I think its not saving the fee.</p>
  
  <p>How can I make it work (on checkout page)?</p>
  
  <p>Thanks</p>