Should you find your Shopp categories and tags urls/slugs not work, it may be because you changed your permalink structure after installing Shopp. To fix this, go to Settings -> Permalinks, and re-save settings. Your categories and tags should work now.
If you use both the excellent WordPress plugins Membership (from WPMU Dev) and Shopp together, you will encounter problems with the PayPal Express payment module in Shopp as they both share a common class name. Symptoms include no payment options in Shopp’s payment settings and the following error message in the logs: mod_fcgid: stderr: PHP Fatal error: Cannot redeclare class PayPalExpress in /wordpressroot/wp-content/plugins/shopp/gateways/PayPal/PayPalExpress.php on line 16, referer: http://domain.com/wp-admin/admin.php?page=shopp-settings-payments The domain and path have obviously been changed. I’ve reported this problem to the Shopp developers so lets see what they say. In the meantime you’ll have to disable Membership until this …