HOWTO: Adding A Discount or Fee to a WooCommerce Order Programmatically

woocommerce-add-fee-to-order.php

<?php

/* 
* The following code assumes you have an WC Order object already. We'll be adding a discount to an order.
* This code cannot be added directly anywhere, it is merely an explanation of how this can be done.
* We will add a discount to the order using Fees 
*/

   /* Assume order is set up already */
  
  //Always save an order first.
  $order->save();

  //Set up the Item Fee
  $fee = new WC_Order_Item_Fee();

  //Give the Fee a name e.g. Discount
  $fee->set_name('Discount');

  //I'll be adding a 50 discount, hence multiply by negative one.
  $total_fee = 50 * -1;

  //Set the Fee
  $fee->set_total($total_fee);

  //Add to the Order
  $order->add_item($fee);

  //Recalculate the totals. IMPORTANT!
  $order->calculate_totals();

  //Save the Order
  $order->save();
  
  //We'll complete the order here, but it doesn't have to be.
  $order->update_status("completed");

?>