Example of accepting drupal commerce coupon via querystring

example.module

<?php

/**
 * Notes: the project this was taken from attempted to limit a user to one coupon per order.
 * Some of the logic was taken directly from commerce_coupon 2.x.
 */

/**
 * Implements hook_init()
 */
function example_commerce_discounts_init() {
  // Grab the query parameters.
  $query_parameters = drupal_get_query_parameters();

  // If we find a coupon code then attempt to redeem it.
  if (!empty($query_parameters['coupon'])) {
    _example_commerce_discounts_redeem_coupon_code($query_parameters['coupon']);
  }
}

/**
 * Custom commerce coupon redeem helper function
 *
 * @param  string $coupon_code
 *   The coupon code that is setup on each coupon.
 * @param  (optional) integer  $order_id
 *   An order ID may optionally be passed. If not passed then attempt to
 *   load the order for the active user.
 */
function _example_commerce_discounts_redeem_coupon_code($coupon_code, $order_id = '') {
  $error = '';
  $uid = $GLOBALS['user']->uid;

  // Load the order based on the order_id or the active user.
  $order = !empty($order_id) ? commerce_order_load($order_id) : commerce_cart_order_load($uid);

  // New users may not have an order to attach a coupon to, so do some extra
  // work to create the order if the previous order load did not retrieve one.
  if (empty($order)) {
    // Try to find the order ID in the session.
    if ($order_id = commerce_cart_order_id($uid)) {
      $order = commerce_order_load($order_id);
    }
    // Create a new order
    else {
      $order = commerce_cart_order_new($uid);
    }
  }

  // Instead of restricting a coupon coming from the querystring from being
  // applied to an order with an existing coupon, it was decided to remove the
  // existing coupon and add the new one.  Leaving the following code commented
  // out in case we need to revert back in the futre.

  // if (_example_commerce_discounts_max_coupons_reached($order, $error)) {
  //   $error = t('User attempted to add more than !count coupons to the cart/order', array('!count' => variable_get_value('example_commerce_discounts_max_coupon_per_order')));
  // }

  // Remove any existing coupons before applying the new one.
  if (!empty($order->commerce_coupons)) {
    $order->commerce_coupons = NULL;
  }

  // Redeem the coupon and get a coupon object in return.
  $coupon = commerce_coupon_redeem_coupon_code($coupon_code, $order, $error);

  // If there is no error and we get a coupon back then reload the order and
  // recalculate the order line items.
  if (empty($error) && !empty($coupon)) {
    // Reload the order so it is not out of date.
    $order = commerce_order_load($order->order_id);

    // Recalculate discounts.
    commerce_cart_order_refresh($order);

    watchdog('commerce_coupon', 'Applied coupon code @code to order !order_id', array('@code' => $coupon_code, '!order_id' => $order->order_id));
  }
  // If there was an error during the redeem process then log the error and
  // remove the coupon from the order.
  elseif (!empty($error)) {
    watchdog('commerce_coupon', 'An error occurred redeeming a coupon: @error', array('@error' => $error));

    if (!empty($coupon)) {
      commerce_coupon_remove_coupon_from_order($order, $coupon);
    }
  }
}