There are some cases when a 10413 error will occur with a PayPal Express Checkout transaction. It seems to be associated mostly with stores who use the tax-included pricing option.

A workaround which has produced moderate success is this:
edit /includes/modules/payment/paypalwpp.php
around line 535 you'll see:
Code:
      $response = $doPayPal->DoExpressCheckoutPayment($_SESSION['paypal_ec_token'],
                                                      $_SESSION['paypal_ec_payer_id'],
                                                      number_format((isset($options['AMT']) ? $options['AMT'] : $order_amount), 2),
                                                      $options);
Change that by adding/editing as follows:
Code:
      if (!isset($options['AMT'])) $options['AMT'] = number_format($order_amount, 2, '.', '');
      $response = $doPayPal->DoExpressCheckoutPayment($_SESSION['paypal_ec_token'],
                                                      $_SESSION['paypal_ec_payer_id'],
                                                      $options['AMT'],
                                                      $options);
Additionally, MAKE SURE you have the ot_subtotal module enabled in Admin->Modules->Order Total.

Fix inspired by a thought by lsenft