Shopping Cart Items Incorrect (Problems with session?)

Hi
Please check out my client’s store at https://francoiseweeks.com/workshops-in-your-area/.
When you put your first item in the cart, everything’s fine. When you put a second item in your cart, only the first item appears. In order to get the second item to appear, you have to delete the first one. If you add more items, it just gets weirder and weirder.
It seems to be storing incorrect info in the session, somehow.
Can you please help?

https://www.ads-software.com/plugins/wordpress-simple-paypal-shopping-cart/