Cannot add more than one item to cart
This is usually caused by your php sessions not being set-up properly.
In most cases this is due to the session save path (php.ini) not set correctly or at all, the directory not existing or not being writable.
Typically this would read
session.save_path = "/tmp" or similar in your php.ini. Refer to the php manual for more details regarding session setup
You should speak to your host / server administrator to get this fixed. WPPizza will not work without session support.