Hello,
The free E-junkie WP plugin exists to simplify the process of adding our Add to Cart and View cart buttons on your site, since Wordpress has a habit of reformatting our code if you just paste it in manually. If you've already successfully added our buttons to your site without the plugin then you're fine, you don't need to use the plugin to add them again. The plugin may save you some time and effort in the future though, or if you find that your original buttons aren't behaving as they should then you might want to delete them and then add them via the plugin.