Add Deposit to WooCommerce Product https://wpsites.net/product-tag/woocommerce_cart_calculate_fees/

Add Deposit to WooCommerce.php

// Add €40 deposit to the cart based on product ID
add_action('woocommerce_cart_calculate_fees', 'add_deposit_fee');
function add_deposit_fee() {
    if (is_admin() && !defined('DOING_AJAX')) return;

    $deposit_fee = 40; // Deposit amount
    $product_ids = array(391); // Replace with your product IDs
    $add_deposit = false;

    foreach (WC()->cart->get_cart() as $cart_item) {
        if (in_array($cart_item['product_id'], $product_ids)) {
            $add_deposit = true;
            break;
        }
    }

    if ($add_deposit) {
        WC()->cart->add_fee(__('Deposit', 'woocommerce'), $deposit_fee);
    }
}

// Add deposit meta to order
add_action('woocommerce_checkout_create_order', 'add_deposit_order_meta', 20, 2);
function add_deposit_order_meta($order, $data) {
    $product_ids = array(391); // Replace with your product IDs
    $add_deposit = false;

    foreach ($order->get_items() as $item_id => $item) {
        if (in_array($item->get_product_id(), $product_ids)) {
            $add_deposit = true;
            break;
        }
    }

    if ($add_deposit) {
        $order->update_meta_data('_deposit_amount', 40);
    }
}