Change "Invoice" to "Receipts"

pmpro_change_Invoice_on_print.php

<?php
/**
 * @link http://codex.wordpress.org/Plugin_API/Filter_Reference/gettext
 * A simple gist that changes "Invoice #:" to "Receipt #:" when printing an exsisting order.
 * Copy the code below into your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_text_strings( $translated_text, $text, $domain ) {
	switch ( $translated_text ) {
		case 'Invoice #: ' :
			$translated_text = __( 'Receipt #: ', 'paid-memberships-pro' );
			break;
	}
	return $translated_text;
}
add_filter( 'gettext', 'my_pmpro_text_strings', 20, 3 );