Great plugin & beautiful Invoices.
I run my local installs with 'WP_DEBUG' set to true in my wp-config.php file, as is recommended for developers: http://codex.wordpress.org/Editing_wp-config.php#Debug
This sets PHP's error reporting level to E_ALL which causes WP3i to throw a bunch of errors. All minor, but errors none the less.
Is the plugin on GitHub? If so, I'll gladly fix the errors and make a pull request.