From orders, it appears that you are calculating the sales tax on an order before the application of a coupon. Customers should be charged sales tax based on the subtotal after coupon application. Either there needs to be a setting where you can say whether to charge before or after coupon application, or this needs to be fixed. I believe the problem is in the: administrator\components\com_easystore\src\Traits\Order.php You're pulling 'discount_type' and 'discount_reason' from the coupon table, but you're not applying them to the sales tax calculation.:

    public function updateOrders()
    {
        $response = (object) [
            'status'  => false,
            'message' => '',
        ];

        $canDo          = ContentHelper::getActions('com_easystore');
        $hasPermission  = $canDo->get('core.create');

        if (!$hasPermission) {
            $this->sendResponse(['message' => Text::_("COM_EASYSTORE_PERMISSION_ERROR_MSG")], 403);
        }

        $id          = $this->getInput('id', 0, 'INT');
        $orderStatus = $this->getInput('order_status', 'draft', 'STRING');

        if (empty($id)) {
            $response->status  = false;
            $response->message = Text::_("COM_EASYSTORE_FAILED_TO_UPDATE_ORDER");

            $this->sendResponse($response);
        }

        $model = new OrderModel();

        $response     = new stdClass();
        $response->id = null;

        $orderInformation = new stdClass();

        $orderInformation->id            = $id;
        $orderInformation->order_status  = $orderStatus;
        $orderInformation->creation_date = null;
        $isActivated                     = false;

        if (!$model->checkCreationDateExists($id) && $orderStatus === 'active') {
            $orderInformation->creation_date = Factory::getDate('now');
            $isActivated                     = true;
        }

        $orderInformation->customer_id      = $this->getInput('customer_id', 0, 'INT');
        $orderInformation->customer_note    = $this->getInput('customer_note', '', 'RAW');
        $orderInformation->payment_status   = $this->getInput('payment_status', 'unpaid', 'STRING');
        $orderInformation->fulfilment       = $this->getInput('fulfilment', 'unfulfilled', 'STRING');
        $orderInformation->discount_type    = $this->getInput('discount_type', 'percent', 'STRING');
        $orderInformation->discount_value   = $this->formatToNumeric($this->getInput('discount_value'), 'decimal');
        $orderInformation->discount_reason  = $this->getInput('discount_reason', '', 'STRING');
        $orderInformation->shipping         = $this->getInput('shipping', '', 'STRING');
        $orderInformation->access           = $this->formatToNumeric($this->getInput('access'));
        $orderInformation->ordering         = $this->formatToNumeric($this->getInput('ordering'));
        $orderInformation->shipping_address = null;
        $orderInformation->billing_address  = null;

        if (!empty($orderInformation->customer_id)) {
            $customerInfo                       = EasyStoreHelper::getCustomerById($orderInformation->customer_id);
            $orderInformation->shipping_address = $customerInfo->shipping_address ?? null;
            $orderInformation->billing_address  = $customerInfo->billing_address ?? null;
        }

        $updateStatus = $model->update($orderInformation);

        if ($updateStatus) {
            $orderProducts = $this->getInput('order_products', '', 'ARRAY');
            $orderProducts = array_map(function ($product) {
                return \json_decode($product, true);
            }, $orderProducts);

            $model->storeMultipleOrderedProducts($orderProducts, $id);

            if (!is_null($orderInformation->shipping_address)) {
                $shippingAddress  = json_decode($orderInformation->shipping_address);
                $country          = $shippingAddress->country;
                $state            = $shippingAddress->state;

                $settingsModel   = new SettingsModel();
                $tax             = $settingsModel->getTaxRate($country, $state);

                $taxRate         = !empty($tax) ? $tax->rate : 0;
                $isTaxOnShipping = !empty($tax) ? $tax->applyOnShipping : false;
                $saleTax         = 0.0;
                $subTotal        = 0.0;
                $calculatedPrice = [];
                foreach ($orderProducts as $product) {
                    $calculatedPrice[] = ($product['discounted_price'] > 0 ? $product['discounted_price'] * $product['quantity'] : $product['price'] * $product['quantity']);
                }
                $subTotal = array_sum($calculatedPrice);
                $saleTax  = ($subTotal * $taxRate) / 100;

                $orderInformation->sale_tax = $saleTax;

                $model->update($orderInformation);
            }

            if ($isActivated) {
                $model->addOrderActivity($id, 'order_created');
            }

            $response->status  = true;
            $response->message = Text::_("COM_EASYSTORE_ORDER_UPDATED");
            $response->id      = $id;
        } else {
            $response->status  = false;
            $response->message = Text::_("COM_EASYSTORE_FAILED_TO_UPDATE_ORDER");
        }

        $this->sendResponse($response);
    }

    I've altered mine as follows:

                foreach ($orderProducts as $product) {
                    $calculatedPrice[] = $product['quantity'] * ($product['discounted_price'] > 0 ? $product['discounted_price'] : $product['price']);
                }
                //foreach ($orderProducts as $product) {
                //  $calculatedPrice[] = ($product['discounted_price'] > 0 ? $product['discounted_price'] * $product['quantity'] : $product['price'] * $product['quantity']);
                //}
                $subTotal = array_sum($calculatedPrice);
                if ($orderInformation->discount_value) {
                  switch ($orderInformation->discount_type) {
                    case 'percent':
                      $subTotal = $subTotal * (100 - $orderInformation->discount_value)/100;
                    case 'amount':
                      $subTotal -= (($subTotal > $orderInformation->discount_value) ? $orderInformation->discount_value : $subTotal);
                    default:
                  }
                } 
                $saleTax  = ($subTotal * $taxRate) / 100;

                $orderInformation->sale_tax = $saleTax;

                $model->update($orderInformation);