code: private function calculatePaymentBase(SupplierOrderInterface $order)
      {
          $base = $this->calculateItemsTotal($order) + $order->getShippingCost() - $order->getDiscountTotal();
  
          $currency = $order->getCurrency()->getCode();
  
          return Money::round($base, $currency);
      }
docs: Calculates the supplier order base.
  
  @param SupplierOrderInterface $order
  
  @return float