I'm having a similar problem. I'm using Cart66 Professional, we're getting an error (readyState 4 status 403) when selecting the product on this page.
At some point it seemed to magically start working properly, however when the product is added to the cart, then deleted, a different product pops up in its place. In this case, a single issue was deleted and a full year subscription appeared. I'm unsure what to do, I thought it might have been a conflict with the WP Super Cache plugin, but I deactivated it and the issue is still there.
Any light you can shine on this would be hugely appreciated!