title: Amazon MWS: getting error 99019 for XML productFeeds (SubmitFeed of type _POST_PRODUCT_DATA_)
body: <p>When I try to add a product (via a SubmitFeed of type "_POST_PRODUCT_DATA_") to the Amazon Market via the Amazon MWS API I get the following error:</p>
  
  <blockquote>
    <p>[Marketplace : Amazon.de] A valid value is required in either the
    "quantity" or "fulfillment_center_id" field.</p>
  </blockquote>
  
  <p>But there is no "quantity" or "fulfillment_center_id" field in my Product.xsd nor in the official documentation. Also the exact same feed does work without an error on my Amazon MWS sandbox account. What am I missing? </p>
  
  <p>Complete XML:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;AmazonEnvelope&gt;
      &lt;Header&gt;
          &lt;DocumentVersion&gt;1.01&lt;/DocumentVersion&gt;
          &lt;MerchantIdentifier&gt;XXX&lt;/MerchantIdentifier&gt;
      &lt;/Header&gt;
      &lt;MessageType&gt;Product&lt;/MessageType&gt;
      &lt;Message&gt;
          &lt;MessageID&gt;1&lt;/MessageID&gt;
          &lt;OperationType&gt;Update&lt;/OperationType&gt;
          &lt;Product&gt;
              &lt;SKU&gt;123456&lt;/SKU&gt;
              &lt;StandardProductID&gt;
                  &lt;Type&gt;EAN&lt;/Type&gt;
                  &lt;Value&gt;767715012826&lt;/Value&gt;
              &lt;/StandardProductID&gt;
              &lt;ProductTaxCode&gt;A_GEN_TAX&lt;/ProductTaxCode&gt;
              &lt;Condition&gt;
                  &lt;ConditionType&gt;New&lt;/ConditionType&gt;
                  &lt;ConditionNote&gt;Zustand/condition: neu OVP&lt;/ConditionNote&gt;
              &lt;/Condition&gt;
              &lt;DescriptionData&gt;
                  &lt;Title&gt;Bellydance for Fitness and Fun&lt;/Title&gt;
                  &lt;Manufacturer&gt;New World 2011&lt;/Manufacturer&gt;
              &lt;/DescriptionData&gt;
              &lt;ProductData&gt;
                  &lt;Music&gt;
                      &lt;ProductType&gt;
                          &lt;MusicPopular&gt;
                              &lt;MediaType&gt;audioCD&lt;/MediaType&gt;
                              &lt;NumberOfDiscs&gt;1&lt;/NumberOfDiscs&gt;
                              &lt;Genre&gt;New Age&lt;/Genre&gt;
                          &lt;/MusicPopular&gt;
                      &lt;/ProductType&gt;
                  &lt;/Music&gt;
              &lt;/ProductData&gt;
          &lt;/Product&gt;
      &lt;/Message&gt;
  &lt;/AmazonEnvelope&gt;
  </code></pre>