WooCommerce auto confirm order code snippet from Kathy. Any order marked for "processing" will be changed to complete without the need to manually click the complete button through the order screen in WooCommerce.

autoconfirm-order-woocommerce.php

add_filter( 'woocommerce_payment_complete_order_status', 'kia_order_payment_complete_order_status', 10, 2 );

function kia_order_payment_complete_order_status( $order_status, $order_id ) {
  $order = wc_get_order( $order_id );

  if ( 'processing' == $order_status &&
       ( 'pending' == $order->status ) ) {

       $order_status = 'completed';
    }

  return $order_status;
}

Place the above code in your child theme functions.php file or create an external php file for WooCommerce code snippets.

From the WooCommerce Slack channel: https://woocommercecommunity.slack.com/messages/C1K9K3UD9/convo/C1K9K3UD9-1515504591.000267/

Another code suggestion for only having virtual products become auto completed: https://www.skyverge.com/blog/how-to-set-woocommerce-virtual-order-status-to-complete-after-payment/

Alternative plugin: https://wordpress.org/plugins/woocommerce-autocomplete-order/