gistfile1.php

<?php
class Foo {
	var $bar;
	
	function Foo() {
		add_action('shopp_init', array($this, 'register_session_variable') );
	}
	
	function register_session_variable() {
		Shopping::restore('bar', $this->bar);
	}
}

$Foo = new Foo();
$Foo->bar = "Cheers"; // this will be saved in the session magically and restored on each new request