亚马逊结账支付按钮集成

我集成了amazon checkout支付按钮代码如下。

<form action="https://authorize.payments-sandbox.amazon.com/pba/paypipeline" method="post">     

<input type="hidden" name="returnUrl" value="http://abcd.com/abcd.php" >

<input type="hidden" name="processImmediate" value="1" >   
  <input type="hidden" name="signatureMethod" value="" >   
  <input type="hidden" name="accessKey" value="" >  
  <input type="hidden" name="collectShippingAddress" value="0" >  
  <input type="hidden" name="isDonationWidget" value="0" >   
  <input type="hidden" name="amazonPaymentsAccountId" value="" >    
  <input type="hidden" name="cobrandingStyle" value="logo" >   
  <input type="hidden" name="immediateReturn" value="1" >   
  <input type="hidden" name="amount" value="USD 10" >   
  <input type="hidden" name="description" value="product payment" >  
  <input type="hidden" name="abandonUrl" value="http://abcd.com/abcd.php" >
  <input type="hidden" name="signatureVersion" value="2" >
  <input type="hidden" name="signature" value="" >   
  <input type="image" src="http://g-ecx.images-amazon.com/images/G/01/asp/beige_small_paynow_withmsg_whitebg.gif" border="0">   
</form>  

数量为<input type="hidden" name="amount" value="USD 10" >当我将其更改为其他值时出现错误，如

无效的请求错误信息:
输入参数签名无效

如何解决这个问题