Add this your customizations plugin to alter text for PMPro Add On Packages.

pmpro-pmproap-text-changes.php

<?php
/**
 * Add to PMPro Customizations plugin. Be sure not to include the opening php tag in line 1.
 *
 * Add this your customizations plugin to alter text for PMPro Add On Packages.
 */
add_action( 'pmpro_invoice_bullets_top', 'pmproap_pmpro_invoice_links_top' );
add_action( 'pmpro_invoice_bullets_top', 'pmproap_pmpro_member_links_top' );
function pmproap_pmpro_invoice_links_top() {
	$invoice_title = 'Purchased Add Ons';
	echo '<h3>' . $invoice_title . '</h3>';
}
function tonys_gettext_pmproap_changes( $translated_text, $text, $domain ) {
	if ( 'paid-memberships-pro' == $domain ) {
		$translated_text = str_replace( 'Membership Level', 'Add On Level Purchase', $translated_text );
		$translated_text = str_replace( 'membership level', 'add on purchase', $translated_text );
		$translated_text = str_replace( 'membership', 'add on purchase', $translated_text );
		$translated_text = str_replace( 'Membership', 'Add On Purchase', $translated_text );
	}
	return $translated_text;
}
add_filter( 'gettext', 'tonys_gettext_pmproap_changes', 10, 3 );