Errors in Debug Mode

I was investigation another problem and activated the debug mode. When I activated your plugin, I got the following warning:

The plugin generated 158 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

I also get the following error displayed on most pages:

Notice: Undefined index: HTTPS in /var/www/vhosts/trilink.aero/mkv/wp-content/plugins/wordpress-simple-paypal-shopping-cart/wp_shopping_cart.php on line 597

Thought that you should know.

http://ww.wp.xz.cn/extend/plugins/wordpress-simple-paypal-shopping-cart/