Default checkout level if no level parameter

default-checkout-level.php

<?php
/**
 * Set default checkout level if none is specified.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_set_default_level( $level ) {
	return 9; // Change the level ID
}
add_filter( 'pmpro_default_level', 'my_pmpro_set_default_level' );