Tell PMPro and PMPro Add On Packages to use the user's current level or the most expensive level when checking out for an add on package.

choose_most_expensive_level_for_addon_packages.php

/**
 * Use new pmproap_text_level_id filter to choose the user's currenet level OR the most expensive level with access.
 * The level with the highest initial payment is chosen.
 * Update this code as needed and add to a custom plugin.
 */
function choose_most_expensive_level_for_addon_packages( $text_level_id, $post_id, $user_id, $post_levels ) {
	if ( ! pmpro_hasMembershipLevel( $text_level_id, $user_id ) ) {
		$highest_initial_payment = 0;
		foreach ( $post_levels as $level_id ) {
			$level = pmpro_getLevel( $level_id );
			if ( $level->initial_payment > $highest_initial_payment ) {
				$highest_initial_payment = $level->initial_payment;
				$text_level_id = $level_id;
			}
		}
	}

	return $text_level_id;
}
add_filter( 'pmproap_text_level_id', 'choose_most_expensive_level_for_addon_packages', 10, 4 );

This recipe is included in the blog post on "Set a Specific Checkout Level When Using Addon Packages" at Paid Memberships Pro here: https://www.paidmembershipspro.com/set-a-specific-checkout-level-when-using-addon-packages/