public function addItem( MShop_Price_Item_Interface $item, $quantity = 1 )
	{
		if( $item->getCurrencyId() != $this->getCurrencyId() )
		{
			throw new MShop_Price_Exception( sprintf( 'Price can not be added. Currency ID "%1$s" of price item and currently used currency ID "%2$s" does not match.', $item->getCurrencyId(), $this->getCurrencyId() ) );
		}

		$this->_values['value'] = $this->_formatNumber( $this->getValue() + $item->getValue() * $quantity );
		$this->_values['costs'] = $this->_formatNumber( $this->getCosts() + $item->getCosts() * $quantity );
		$this->_values['rebate'] = $this->_formatNumber( $this->getRebate() + $item->getRebate() * $quantity );
	}