Dias a mais para produtos sob encomenda.

functions.php

<?php

add_filter('woocommerce_correios_shipping_additional_time', function($additional_time, $package){

    $has_outofstock_item = false;

    foreach( $package['contents'] as $item ) {
        if ( 'onbackorder' === wc_get_product($item['product_id'])->get_stock_status() ){
            $has_outofstock_item = true;
            break;
        }
    }

    if( $has_outofstock_item ) {
        $additional_time = 3; // tempo adicional
    }

    return $additional_time;

}, 10, 3);