Common/Model/Notify.php
public function addLabel(ShipmentLabelInterface $label)
    {
        if (!$this->labels->contains($label)) {
            $this->labels->add($label);
        }

        return $this;
    }
public function addLabel(ShipmentLabelInterface $label)
    {
        if (!$this->labels->contains($label)) {
            $this->labels->add($label);
        }

        return $this;
    }
Adds the label.

@param ShipmentLabelInterface $label

@return Notify