Plugin Allowing to Add Other Products to Cart

Hi. I am using this plugin on the following site: https://hedgehogcity.com

Due to my shipping companies, if a customer wants to buy this t-shirt, I want them to not be able to add anything else to the cart: https://hedgehogcity.com/shop/t-shirts/halloween-hedgehog-frankenstein-t-shirt-franken-hedge/

The plugin works if you have other products in your cart, and then try to add the shirt.

However, if I add the shirt to my cart first, I am then able to add other products, which should not be allowable.