code: private function calculateAdjustment(AdjustmentDataInterface $adjustment, $base)
      {
          if ($adjustment->getMode() === AdjustmentModes::MODE_PERCENT) {
              return Money::round($base * $adjustment->getAmount() / 100, $this->currency);
          }
  
          if ($adjustment->getMode() === AdjustmentModes::MODE_FLAT) {
              return $adjustment->getAmount();
          }
  
          throw new InvalidArgumentException("Unexpected adjustment mode.");
      }
docs: Calculates the adjustment amount.
  
  @param AdjustmentDataInterface $adjustment
  @param                         $base
  
  @return float