En Woocommerce, uso campos personalizados para calcular el precio de un producto, basado en este código de hilo:Display campos personalizados de productos como artículos de pedido en Woocommerce 3.

// Add a custom field before single add to cart
add_action('woocommerce_before_add_to_cart_button', 'custom_product_price_field', 5);

function custom_product_price_field() {
echo '<div class="custom-text text">
<h3>Rental</h3>
<label>Start Date:</label>
<input type="date" name="rental_date" value="" class="rental_date" />
<label>Period Rental:</label>
<select name="custom_price" class="custom_price">
    <option value="" selected="selected">choosen period</option>
    <option value="2">2 days</option>
    <option value="4">4 days</option>
</select>
</div>';
}

// Get custom field value, calculate new item price, save it as custom cart item data
add_filter('woocommerce_add_cart_item_data', 'add_custom_field_data', 20, 3);

function add_custom_field_data($cart_item_data, $product_id, $variation_id) {
if (isset($_POST['rental_date']) && !empty($_POST['rental_date'])) {
        $cart_item_data['custom_data']['date'] = $_POST['rental_date'];
}
if (isset($_POST['custom_price']) && !empty($_POST['custom_price'])) {
        $_product_id = $variation_id > 0 ? $variation_id : $product_id;
        $product = wc_get_product($_product_id); // The WC_Product Object
        $base_price = (float) $product - > get_regular_price(); // Product reg price
        $custom_price = (float) sanitize_text_field($_POST['custom_price']);

        $cart_item_data['custom_data']['base_price'] = $base_price;
        $cart_item_data['custom_data']['new_price'] = $base_price/100 * 15 * $custom_price;
        $cart_item_data['custom_data']['rental'] = $custom_price;
}
if (isset($cart_item_data['custom_data']['new_price']) || isset($cart_item_data['custom_data']['date'])) {
        $cart_item_data['custom_data']['unique_key'] = md5(microtime().rand()); // Make each item unique
}
return $cart_item_data;
}

// Set the new calculated cart item price
add_action('woocommerce_before_calculate_totals', 'extra_price_add_custom_price', 20, 1);

function extra_price_add_custom_price($cart) {
if (is_admin() && !defined('DOING_AJAX'))
        return;

foreach($cart - > get_cart() as $cart_item) {
        if (isset($cart_item['custom_data']['new_price']))
                $cart_item['data'] - > set_price((float) $cart_item['custom_data']['new_price']);
}
}

// Display cart item custom price details
add_filter('woocommerce_cart_item_price', 'display_cart_items_custom_price_details', 20, 3);

function display_cart_items_custom_price_details($product_price, $cart_item, $cart_item_key) {
if (isset($cart_item['custom_data']['base_price'])) {
        $product = $cart_item['data'];
        $base_price = $cart_item['custom_data']['base_price'];
        $product_price = wc_price(wc_get_price_to_display($product, array('price' => $base_price))).
        '<br>';
        if (isset($cart_item['custom_data']['rental'])) {
                $product_price. = $cart_item['custom_data']['rental'] == '2' ? __("2 days") : __("4 days");
        }
}
return $product_price;
}

// Display in cart item the selected date
add_filter('woocommerce_get_item_data', 'display_custom_item_data', 10, 2);

function display_custom_item_data($cart_item_data, $cart_item) {
if (isset($cart_item['custom_data']['date'])) {

        $cart_item_data[] = array(
                'name' => __("Chosen date", "woocommerce"),
                'value' => date('d.m.Y', strtotime($cart_item['custom_data']['date'])),
        );
}
if (isset($cart_item['custom_data']['rental'])) {
        $cart_item_data[] = array(
                'name' => __("Period Rental", "woocommerce"),
                'value' => $cart_item['custom_data']['rental'] == '2' ? __("2 days") : __("4 days"),
        );
}
return $cart_item_data;
}

Es necesario cambiar las condiciones para calcular el nuevo precio. Por el momento, el cálculo del nuevo precio no tiene en cuenta la cantidad de días. Esta es una condición necesaria.

Si el usuario selecciona "2 días", el cálculo será ...$base_price/100 * 15 * value=2

Si el usuario selecciona "4 días", el cálculo será ...$base_price/100 * 15 * value=4

¿Cómo puedo hacer eso

ACTUALIZAR Lo siento, olvidé agregar el último código que me diste. ¿Cómo estar con él?

// Save and display custom field in orders and email notifications (everywhere)
add_action( 'woocommerce_checkout_create_order_line_item', 'custom_fields_update_order_item_meta', 20, 4 );
function custom_fields_update_order_item_meta( $item, $cart_item_key, $values, $order ) {
if ( isset( $values['custom_data']['date'] ) ){
    $date = date( 'd.m.Y', strtotime( $values['custom_data']['date'] ) );
    $item->update_meta_data( __( 'Choosen Date', 'woocommerce' ), $date );
}
if ( isset( $values['custom_data']['rental'] ) ){
    $rental = $values['custom_data']['rental'] == '2' ? __("2 days") : __("4 days");
    $item->update_meta_data( __( 'Period Rental', 'woocommerce' ), $rental );
}
}