Why am I getting an Undefined Index

public function addTransaction($customer_id, $description = '',  $amount = '', $N2Ugiven = '', $N2Ufrom = '', $order_id = 0) {
    $customer_info = $this->getCustomer($customer_id);

    if ($customer_info) {
        $this->db->query("INSERT INTO " . DB_PREFIX . "customer_transaction SET customer_id = '" . (int)$customer_id . "', order_id = '" . (int)$order_id . "', description = '" . $this->db->escape($description) . "', amount = '" . (float)$amount . "', N2Ugiven = '" . $this->db->escape($N2Ugiven) . "',  N2Ufrom = '" . $this->db->escape($N2Ufrom) . "', date_added = NOW()");

    if (($this->request->server['REQUEST_METHOD'] == 'POST') && $this->user->hasPermission('modify', 'sale/customer')) {
        $this->model_sale_customer->addTransaction($this->request->get['customer_id'], $this->request->post['description'], $this->request->post['N2Ufrom'], $this->request->post['N2Ugiven'], $this->request->post['amount']);

        $data['success'] = $this->language->get('text_success');
    } else {
        $data['success'] = '';
    }

    if (($this->request->server['REQUEST_METHOD'] == 'POST') && !$this->user->hasPermission('modify', 'sale/customer')) {
        $data['error_warning'] = $this->language->get('error_permission');
    } else {
        $data['error_warning'] = '';
    }

    $data['text_no_results'] = $this->language->get('text_no_results');
    $data['text_balance'] = $this->language->get('text_balance');

    $data['column_date_added'] = $this->language->get('column_date_added');
    $data['column_N2Ugiven'] = $this->language->get('column_N2Ugiven');
    $data['column_N2Ufrom'] = $this->language->get('column_N2Ufrom');
    $data['column_description'] = $this->language->get('column_description');
    $data['column_amount'] = $this->language->get('column_amount');

    if (isset($this->request->get['page'])) {
        $page = $this->request->get['page'];
    } else {
        $page = 1;
    }

    $data['transactions'] = array();

    $results = $this->model_sale_customer->getTransactions($this->request->get['customer_id'], ($page - 1) * 10, 10);

    foreach ($results as $result) {
        $data['transactions'][] = array(
            'amount'      => $this->currency->format($result['amount'], $this->config->get('config_currency')),
            'N2Ugiven'    => $result['N2Ugiven'],
            'N2Ufrom'     => $result['N2Ufrom'],
            'description' => $result['description'],
            'date_added'  => date($this->language->get('date_format_short'), strtotime($result['date_added']))
        );  
    }