protected function setShippingDiscount($data)
    {
        if (isset($data['shipping_discount'])) {
            if ($data['shipping_discount'] > 0) {
                $data['shipping_discount'] *= -1;
            }

            $this->post = $this->post->merge([
                'PAYMENTREQUEST_0_SHIPDISCAMT' => $data['shipping_discount'],
                'PAYMENTREQUEST_0_AMT'         => round($data['total'] + $data['shipping_discount'], 2),
            ]);
        }
    }