Send invoice to admin on order status pending to processing

functions.php

<?php

add_action( 'woocommerce_order_status_pending_to_processing_notification', function( $order_id ) {
    if ( $order = sab_get_order( $order_id ) ) {
        if ( $invoice = $order->get_latest_finalized_invoice() ) {
            $invoice->send_to_admin();
        }
    }
}, 10 );