解決辦法：
找到文件 app/services/order/StoreOrderDeliveryServices.php
在最后加上這段代碼 （最后一個}里）

/**
     * 返回訂單商品總重量
     * @param int $id
     * @return int|string
     */
    public function getOrderSumWeight(int $id, $default = false)
    {
        /** @var StoreOrderCartInfoServices $services */
        $services = app()->make(StoreOrderCartInfoServices::class);
        $orderGoodInfo = $services->getOrderCartInfo((int)$id);
        $weight = 0;
        foreach ($orderGoodInfo as $cartInfo) {
            $cart = $cartInfo['cart_info'] ?? [];
            if ($cart) {
                $weight = bcadd((string)$weight, (string)bcmul((string)$cart['cart_num'] ?? '0', (string)$cart['productInfo']['attrInfo']['weight'] ?? '0', 4), 2);
            }
        }
        return $weight ? $weight : ($default === false ? 0 : $default);
    }