Make sure that the MemberInvoice class is loaded when using MMPU.

ensure_MemberInvoice_loaded.php

<?php

// Copy from below here...

/**
 * Make sure that the MemberInvoice class is loaded when using MMPU.
 * 
 * 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_ensure_MemberInvoice_loaded() {
	if ( ! class_exists( 'MemberInvoice' ) && defined( 'PMPROMMPU_DIR' ) && file_exists( PMPROMMPU_DIR . '/includes/invoice.php' ) ) {
		require PMPROMMPU_DIR . '/includes/invoice.php';
	}
}
add_action( 'init', 'my_pmpro_ensure_MemberInvoice_loaded' );