Mark Cash Payment as Paid

payment-cash.php

add_action( 'woocommerce_thankyou', 'wc_auto_complete_paid_order' );
function wc_auto_complete_paid_order( $order_id ) {

  if ( ! $order_id ) {
    return;
  }

  $order = wc_get_order( $order_id );

  if( $order->has_status('processing') ) {
    $order->update_status( 'completed' );
  } else {
    $order->update_status( 'cancelled' );
  }

}