Add an expiration date to orders based on order items count in WooCommerce on your child site.
Snippet Type
Execute on Child Sites
Snippet
add_action( 'woocommerce_checkout_create_order', 'add_expiration' );
function add_expiration( $order, $data ) {
$items_count = WC()->cart->get_cart_contents_count();
$order->update_meta_data('expiration_date', date( 'Y-m-d H:i:s', strtotime( '+'. ( $items_count * 15 ) .' days' ) ) );
}