Change the text for the addon package checkout page to only show the post title and exclude the level name.

change-text-for-addon-package-checkout.php

<?php
/**
 * Change the text for the addon package checkout page.
 *
 * 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/
 */
// Swap the pmproap_pmpro_checkout_level filter with our custom one.
function init_swap_pmproap_pmpro_checkout_level() {
	if ( ! function_exists( 'pmproap_pmpro_checkout_level' ) || ! function_exists( 'my_pmproap_pmpro_checkout_level' ) ) {
		return;
	} else {
		remove_filter( 'pmpro_checkout_level', 'pmproap_pmpro_checkout_level' );
		add_filter( 'pmpro_checkout_level', 'my_pmproap_pmpro_checkout_level' );
	}
}
add_action( 'init', 'init_swap_pmproap_pmpro_checkout_level' );

// Tweak the checkout page when ap is passed in.
function my_pmproap_pmpro_checkout_level( $level ) {
	global $current_user;

	if ( ! function_exists( 'pmproap_pmpro_checkout_level' ) || ! function_exists( 'my_pmproap_pmpro_checkout_level' ) ) {
		return $level;
	}

	if ( ! isset( $level->id ) ) {
		return $level;
	}

	// are we purchasing a post?
	if ( isset( $_REQUEST['ap'] ) && ! empty( $_REQUEST['ap'] ) ) {
		$ap            = intval( $_REQUEST['ap'] );
		$ap_post       = get_post( $ap );
		$pmproap_price = get_post_meta( $ap, '_pmproap_price', true );

		if ( ! empty( $pmproap_price ) ) {
			if ( pmpro_hasMembershipLevel( $level->id ) ) {
				// already have the membership, so price is just the ap price
				$level->initial_payment = $pmproap_price;

				// zero the rest out
				$level->billing_amount = 0;
				$level->cycle_number   = 0;
				$level->trial_amount   = 0;
				$level->trial_limit    = 0;

				// unset expiration period and number
				$level->expiration_period = null;
				$level->expiration_number = null;
			} else {
				// add the ap price to the membership
				$level->initial_payment = $level->initial_payment + $pmproap_price;
			}

			// update the name
			$level->name = $ap_post->post_title;
		} else {
			// woah, they passed a post id that isn't locked down
		}
	}

	return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmproap_pmpro_checkout_level' );

// Change the text for the addon package checkout page.
function my_pmpro_change_text_for_addon_package_checkout( $translated_text, $text, $domain ) {
	if ( ! function_exists( 'pmproap_pmpro_checkout_level' ) || ! function_exists( 'my_pmproap_pmpro_checkout_level' ) ) {
		return $translated_text;
	}
	if ( ! empty( $_REQUEST['ap'] ) || ( ! empty( $_REQUEST['level'] ) || ! empty( $_REQUEST['pmpro_level'] ) ) ) {
		if ( 'paid-memberships-pro' === $domain && 'You have selected the %s membership level.' === $text ) {
			$translated_text = __( 'You have selected to purchase access to the post: %s', 'paid-memberships-pro' );
		}
	}
	return $translated_text;
}

add_filter( 'gettext', 'my_pmpro_change_text_for_addon_package_checkout', 20, 3 );