public function applyFlatDiscount($amount, $couponId = null)
    {
        if ($amount > $this->subtotal) {
            throw new IncorrectDiscount('The discount amount cannot be more that subtotal of the cart');
        }

        $this->discount = round($amount, 2);
        $this->discountPercentage = 0;
        $this->couponId = $couponId;

        $cartData = $this->cartUpdates($isNewItem = false, $keepDiscount = true);

        event(new DiscountApplied($cartData));

        return $cartData;
    }