Only one item shows in shopping cart at a time

I’m 99% complete with a clients new website, with the only thing left being getting a simple shopping cart up. This plugin looks great, but the shopping cart only shows one item even if I try putting in multiple items. It always shows the last item I added but none of the previous items. Any idea what’s going on? I’m not running any caching software/plugins. Thanks for any suggestions!

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