Can't uninstall Rakuten extension

I thought I'd try installing, using, then uninstalling the Rakuten Cash-back extension. Unfortunately, when I went to safari>preferences>extension>Rakuten>uninstall I get a message "To uninstall “Rakuten Cash Back”, you must remove the “Rakuten Cash Back” application.". I've not only deleted the application, I also deleted the rakuten user library on my computer, which was the only thing that came up in a search of the HD and of the Macbook Pro, then emptied the trash, emptied the cache of the Rakuten website, and....it's still there under extensions, and I get the same message when clicking on "uninstall". I am running Safari 13.1, have all the latest updates installed. OS 10.13.6