Add username to PayPal order description. Equivalent for Authorize.net also included, but commented out.

my_pmpro_custom_level_description_add_username.php

<?php

// Copy from below here...

/**
 * Add username to PayPal order description.
 * Equivalent for Authorize.net also included, but commented out.
 */
function my_pmpro_custom_level_description_add_username( $description, $level_name, $order, $site_name ) {
	//Initialize $username
	$username = '';

	// Get username for current user
	if ( array_key_exists( 'username', $_REQUEST ) ) {
		// First checkout.
		$username = $_REQUEST['username'];
	} elseif ( array_key_exists( 'pmpro_signup_username', $_SESSION ) ) {
		// Returning from PayPal during thier first checkout.
		$username = $_SESSION['pmpro_signup_username'];
	} else {
		// Existing user.
		$user = wp_get_current_user();
		if ( ! empty( $user ) && isset( $user->user_login ) ) {
			$username = $user->user_login;
		}
	}

	// Check that we've found a username
	if ( empty( $username ) )  {
		// We should never get here.
		echo "Couldn't get a username.";
		wp_die();
	}

	// Format description.
	$description = $username . ': ' . $description;
	return substr( $description, 0, 127);
}
add_filter( 'pmpro_paypal_level_description', 'my_pmpro_custom_level_description_add_username', 10, 4 );
// add_filter( 'pmpro_authorizenet_level_description', 'my_pmpro_custom_level_description_add_username', 10, 4 );