Paypal API: The totals of the cart item amounts do not match order amounts

Question

I am working with the paypal express checkout API and am having issues with setting the quantity. Below are my URI parameters:

$params = array(
   'METHOD' => 'SetExpressCheckout',
   'PAYMENTREQUEST_0_SHIPPINGAMT' => '0',
   'PAYMENTREQUEST_0_TAXAMT' => '0',
   'PAYMENTREQUEST_0_HANDLINGAMT' => '0',
   'PAYMENTREQUEST_0_AMT' => "50.00",
   'RETURNURL' => $this->_returnUri,
   'CANCELURL' => $this->_cancelUri,
   'HDRIMG' => $this->_logoUri,
   'PAYMENTREQUEST_0_CURRENCYCODE' => 'GBP',
   'L_PAYMENTREQUEST_0_DESC0' => 'Product description',
   'L_PAYMENTREQUEST_0_NAME0' => 'Product name',
   'L_PAYMENTREQUEST_0_AMT0' => "50.00",
   'L_PAYMENTREQUEST_0_QTY0' => "2"
);

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:

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

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 ?

Answer 1

You're missing an ITEMAMT. See also my answer in Paypal Express checkout with shipping cost and error which contains a working example with multiple quantities.

&L_PAYMENTREQUEST_0_NAME0=10% Decaf Kona Blend Coffee  
&L_PAYMENTREQUEST_0_NUMBER0=623083  
&L_PAYMENTREQUEST_0_DESC0=Size: 8.8-oz 
&L_PAYMENTREQUEST_0_AMT0=9.95  
&L_PAYMENTREQUEST_0_QTY0=2  
&L_PAYMENTREQUEST_0_NAME1=Coffee Filter bags  
&L_PAYMENTREQUEST_0_NUMBER1=623084  
&L_PAYMENTREQUEST_0_DESC1=Size: Two 24-piece boxes  
&L_PAYMENTREQUEST_0_AMT1=39.70  
&L_PAYMENTREQUEST_0_QTY1=2  
&PAYMENTREQUEST_0_ITEMAMT=99.30  
&PAYMENTREQUEST_0_TAXAMT=2.58  
&PAYMENTREQUEST_0_SHIPPINGAMT=3.00  
&PAYMENTREQUEST_0_HANDLINGAMT=2.99  
&PAYMENTREQUEST_0_SHIPDISCAMT=-3.00  
&PAYMENTREQUEST_0_INSURANCEAMT=1.00  
&PAYMENTREQUEST_0_AMT=105.87  
&PAYMENTREQUEST_0_CURRENCYCODE=USD