<?php
/* Etiquetas de los campos dentro del campo al finalizar compra */
add_filter( 'woocommerce_checkout_fields', 'etiquetas_dentro_finalizar_compra', 9999 );
function etiquetas_dentro_finalizar_compra( $fields ) {
    foreach ( $fields as $section => $section_fields ) {
        foreach ( $section_fields as $section_field => $section_field_settings ) {
            $fields[$section][$section_field]['placeholder'] = $fields[$section][$section_field]['label'];
            $fields[$section][$section_field]['label'] = '';
        }
    }
    return $fields;
}