I also have a shared SSL cert and ran into the same problem. After some trial and error, I found that if I encrypted my entire cart, things seemed to work. In your case:

define('HTTP_SERVER', 'https://ssl.perfora.net/www.myshop.net');
define('HTTPS_SERVER', 'https://ssl.perfora.net/www.myshop.net');

// Use secure webserver for checkout procedure?
define('ENABLE_SSL', 'true');


My application isn't overly intensive, so there wasn't much performance loss and it now allows my customers to check out via PayPal express.

Cheers.