"jQuery Not Defined" Error When Connecting to PayPal

I have been looking all over Google for hours now and am very confused. Everyone has a different answer, and most don't explain where to put the code snippet that they post.

I am attempting to create a new blog and web store with a self-hosted WordPress site. The site is running on a machine running the latest version of Ubuntu Server and apache2. I am using the plugin WP Simple PayPal Shopping Cart to create a way for users to checkout. On the checkout page, attempting to checkout with PayPal Express will cause the ReferenceError: jQuery is not defined error, seemingly due to a JavaScript file on PayPal's servers.

I am not sure how to fix this. I am aware that jQuery should be loaded before anything attempts to run it, but I have no idea how to do that, or if it's already correct and that isn't the issue at all. I also saw that some speed boosting plugins could cause the issue but I have not installed any of them. Any help would be appreciated.

The site is broketrainenthusiast.com but to see the issue you'll need to add this fake item to your cart at broketrainenthusiast.com/example/ and then initiate a purchase by clicking the PayPal button on broketrainenthusiast.com/cart/ (don't worry. it's in Sandbox mode).

As I said, any help with this would be appreciated, as this error prevents any successful transaction from occurring.