配送料が0だと合計計算されない

配送料が0円だと、なぜか合計金額が計算されません。
これは仕様なのかも知れませんが、釈然としません。そこで、core/orders.php の

if( !empty( $this->aOrders[$iId]['fPaymentCarrierPrice'] ) ){
$this->aOrders[$iId]['fOrderSummary'] = $aData['fOrderSummary'] = 
normalizePrice( $aData['fProductsSummary'] + 
$this->aOrders[$iId]['fPaymentCarrierPrice'] );
$this->aOrders[$iId]['sOrderSummary'] = $aData['sOrderSummary'] = 
displayPrice( $aData['fOrderSummary'] );
}

を

$this->aOrders[$iId]['fOrderSummary'] = $aData['fOrderSummary'] = 
normalizePrice( $aData['fProductsSummary'] + 
$this->aOrders[$iId]['fPaymentCarrierPrice'] );
$this->aOrders[$iId]['sOrderSummary'] = $aData['sOrderSummary'] = 
displayPrice( $aData['fOrderSummary'] );

としてみました。