/**
  * set the shipping chargetype on the supplied shipgroup payload
  * @param  IShipGroup $shipGroup
  * @return self
  */
 public function setShippingChargeType(IShipGroup $shipGroup)
 {
     // Use flatrate because the way shipping costs are calculated
     // in Magento makes it impossible to determine how much each
     // item contributes to the shipping cost.
     $shipGroup->setChargeType(self::SHIPPING_CHARGE_TYPE_FLATRATE);
     return $this;
 }