Adds a field to the Commerce checkout, such as a sales rep.

mymodule.module

<?php

/**
 * Implements hook_commerce_checkout_pane_info().
 */
function module_commerce_checkout_pane_info() {

  $panes['module_salesrep'] = array(
    'title' => t('Sales Representative'),
    'base' => 'module_salesrep',

    // The checkout page where this should be displayed by default.
    'page' => 'checkout',
    'weight' => -19,
  );

  return $panes;
}

/**
 * Account pane: settings form callback.
 *
 * This form provides configuration information for the pane. In this case
 * we set a variable determining whether to request an additional email address.
 */
function module_salesrep_settings_form($checkout_pane) {
  $form = array();

  return $form;
}

/**
 * Example Pane: form callback.
 *
 * This is a standard FAPI form which will be presented in the pane.
 * The form gathers and stores information from $order->data, an array which
 * can be populated with free-form keys and values.
 */
function module_salesrep_checkout_form($form, &$form_state, $checkout_pane, $order) {

  $pane_form['sales_rep_name'] = array(
    '#type' => 'textfield',
    '#title' => t('Sales Representative Name'),
    '#description' => t("Please enter the name of your sales representative, if available."),
    // If using field on user, load an order wrapper &.... $order_wrapper->uid->field_sales_rep_name->value();
    '#default_value' => !empty($order->data['sales_rep_name']) ? $order->data['sales_rep_name'] : '',
    '#required' => FALSE,
  );

  return $pane_form;
}

/**
 * Example pane: checkout form submission callback.
 */
function module_salesrep_checkout_form_submit($form, &$form_state, $checkout_pane, $order) {
  if (!empty($form_state['values'][$checkout_pane['pane_id']])) {
    $pane_values = $form_state['values'][$checkout_pane['pane_id']];

    if (!empty($pane_values['sales_rep_name'])) {
      // If using a field on user, load wrapper && $order_wrapper->uid->field_sales_rep_name->set($pane...);
      $order->data['sales_rep_name'] = $pane_values['sales_rep_name'];
    }
  }
}

/**
 * Example pane: presents the information we've collected in the Review checkout
 * pane.
 */
function module_salesrep_review($form, $form_state, $checkout_pane, $order) {
  if (!empty($order->data['sales_rep_name'])) {
    $content['sales_rep_name'] = array(
      '#type' => 'item',
      '#title' => t('Sales Representative Name'),
      // If using order... $order_wrapper->uid->field_sales_rep_name
      '#markup' => check_plain($order->data['sales_rep_name']),
    );
  }
  return drupal_render($content);
}