As a quick fix, you can apply the following solution to your store.
Edit the classes/XLite/Module/CDev/Paypal/Model/Payment/Processor/PaypalWPS.php file and add this line:
PHP Code:
$orderNumber = $this->getSetting('prefix') . $order->getOrderNumber();
after this one:
PHP Code:
$orderTotal = $currency->roundValue($order->getTotal());
Then, replace this line:
PHP Code:
'invoice' => $orderTotal,
with the next one around line number 409:
PHP Code:
'invoice' => $orderNumber,
After that re-deploy the store and the problem will be fixed.
Tony.