title: Amazon MWS API - SubmitFeed completed but quantity not updated
body: <p>I need to update quantity for a single product via SubmitFeed API and "_POST_INVENTORY_AVAILABILITY_DATA_" feed type:</p>
  
  <pre><code>&lt;?xml version=""1.0"" encoding=""utf-8"" ?&gt;
          &lt;AmazonEnvelope xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xsi:noNamespaceSchemaLocation=""amznenvelope.xsd""&gt;
              &lt;Header&gt;
                  &lt;DocumentVersion&gt;1.01&lt;/DocumentVersion&gt;
                  &lt;MerchantIdentifier&gt;MERCHANT_ID&lt;/MerchantIdentifier&gt;
              &lt;/Header&gt;
              &lt;MessageType&gt;Inventory&lt;/MessageType&gt;
              &lt;Message&gt;
                  &lt;MessageID&gt;1&lt;/MessageID&gt;
                  &lt;OperationType&gt;Update&lt;/OperationType&gt;
                  &lt;Inventory&gt;
                      &lt;SKU&gt;MY_SKU_CODE&lt;/SKU&gt;
                      &lt;Quantity&gt;2&lt;/Quantity&gt;
                  &lt;/Inventory&gt;
              &lt;/Message&gt;
          &lt;/AmazonEnvelope&gt;
  </code></pre>
  
  <p>Submission was OK (checked from MWS Scratchpad):</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;AmazonEnvelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="amzn-envelope.xsd"&gt;
      &lt;Header&gt;
          &lt;DocumentVersion&gt;1.02&lt;/DocumentVersion&gt;
          &lt;MerchantIdentifier&gt;...&lt;/MerchantIdentifier&gt;
      &lt;/Header&gt;
      &lt;MessageType&gt;ProcessingReport&lt;/MessageType&gt;
      &lt;Message&gt;
          &lt;MessageID&gt;1&lt;/MessageID&gt;
          &lt;ProcessingReport&gt;
              &lt;DocumentTransactionID&gt;...&lt;/DocumentTransactionID&gt;
              &lt;StatusCode&gt;Complete&lt;/StatusCode&gt;
              &lt;ProcessingSummary&gt;
                  &lt;MessagesProcessed&gt;1&lt;/MessagesProcessed&gt;
                  &lt;MessagesSuccessful&gt;1&lt;/MessagesSuccessful&gt;
                  &lt;MessagesWithError&gt;0&lt;/MessagesWithError&gt;
                  &lt;MessagesWithWarning&gt;0&lt;/MessagesWithWarning&gt;
              &lt;/ProcessingSummary&gt;
          &lt;/ProcessingReport&gt;
      &lt;/Message&gt;
  &lt;/AmazonEnvelope&gt;
  </code></pre>
  
  <p>but the quantity for that SKU hadn't changed in my inventory (as seen in my Amazon Seller Central back office). Amazon's forums did not help. Any suggestion?</p>