I'm pretty sure this is related to the hosting setup but just FYI:
I installed this on a client's site and it didn't work. Every time I clicked on add to cart, it would add to the cart ONCE. But go to another page and the cart was empty again. It wasn't remembering any entered items.
After searching around, I found a support document on the host's web site, saying you need to designate the path for PHP sessions.
I inserted this line (edited for anonymity):
just before session_start();
And it works.
So while the problem is solved, I do wonder if there's a workaround that doesn't require me to edit the plugin file.