Adiciona custom fields ao woocommerce sem ACF

woocommerce-custom-fields.php

<?php
// Caggera os campos nas opções de preço do woocommerce.
// https://rudrastyh.com/woocommerce/product-data-metabox.html
// https://stackoverflow.com/questions/43700927/display-and-save-a-custom-field-in-product-variations-options-edit-page
add_action('woocommerce_product_options_pricing', function() {
    global $woocommerce, $post;
    
    $price_monthly = get_post_meta( $post->ID, 'price-monthly', true );
    woocommerce_wp_text_input([
        'id' => 'price-monthly',
        'label' => 'Preço mensal',
        'data_type' => 'price',
    ], $price_monthly ? $price_monthly : '');

    $price_monthly_promotional = get_post_meta( $post->ID, 'price-monthly-promotional', true );
    woocommerce_wp_text_input([
        'id' => 'price-monthly-promotional',
        'label' => 'Preço mensal promocional',
        'data_type' => 'price',
    ], $price_monthly_promotional ? $price_monthly_promotional : '');
});

// Salva os valores dos campos acima na postmeta.
add_action('woocommerce_process_product_meta', function($post_id) {
    $price_monthly = $_POST['price-monthly'];
    if (isset($price_monthly)) {
        update_post_meta($post_id, 'price-monthly', esc_attr($price_monthly));
    }

    $price_monthly_promotional = $_POST['price-monthly-promotional'];
    if (isset($price_monthly_promotional)) {
        update_post_meta($post_id, 'price-monthly-promotional', esc_attr($price_monthly_promotional));
    }
});