gistfile1.php

<?php

//...

	public function filter_default_rewrite_rules( $rules ) {

		$rules[] = array(
			'name'			=>	'display_invoice',
			'parse_regex'	=>	'%^(?P<client_slug>.+?)/invoice/(?P<slug>.+?)/?$%i',
			'build_str'		=>	'{$client_slug}/invoice/{$slug}',
			'handler'		=>	'UserThemeHandler',
			'action'		=>	'display_post',
			'priority'		=>	1,
			'description'	=>	'Display Invoice',
			'parameters' => serialize( array( 'require_match' => array('Posts', 'rewrite_match_type'), 'content_type'=>'invoice') ),
		);

		return $rules;
	}