title: Paypal API: The totals of the cart item amounts do not match order amounts
body: <p>I am working with the paypal express checkout API and am having issues with setting the quantity. Below are my URI parameters:</p>
  
  <pre><code>$params = array(
     'METHOD' =&gt; 'SetExpressCheckout',
     'PAYMENTREQUEST_0_SHIPPINGAMT' =&gt; '0',
     'PAYMENTREQUEST_0_TAXAMT' =&gt; '0',
     'PAYMENTREQUEST_0_HANDLINGAMT' =&gt; '0',
     'PAYMENTREQUEST_0_AMT' =&gt; "50.00",
     'RETURNURL' =&gt; $this-&gt;_returnUri,
     'CANCELURL' =&gt; $this-&gt;_cancelUri,
     'HDRIMG' =&gt; $this-&gt;_logoUri,
     'PAYMENTREQUEST_0_CURRENCYCODE' =&gt; 'GBP',
     'L_PAYMENTREQUEST_0_DESC0' =&gt; 'Product description',
     'L_PAYMENTREQUEST_0_NAME0' =&gt; 'Product name',
     'L_PAYMENTREQUEST_0_AMT0' =&gt; "50.00",
     'L_PAYMENTREQUEST_0_QTY0' =&gt; "2"
  );
  </code></pre>
  
  <p>If L_PAYMENTREQUEST_0_QTY0 is set to 1 the process is fine. However, changing it to 2 and adding 'PAYMENTREQUEST_0_ITEMAMT' => "100.00" to the array gives me the following error:</p>
  
  <pre><code>TIMESTAMP: 2012-04-17T12:11:16Z
  CORRELATIONID: da43f6c91254
  ACK: Failure
  VERSION: 88.0
  BUILD: 2808426
  L_ERRORCODE0: 10413
  L_SHORTMESSAGE0: Transaction refused because of an invalid argument. See additional error messages for details.
  L_LONGMESSAGE0: The totals of the cart item amounts do not match order amounts.
  L_SEVERITYCODE0: Error
  </code></pre>
  
  <p>I have tried for hours on end to fix it and have given up. Can someone highlight want I need to do to get it to work ?</p>