I found some small bugs in the plugin that I was wondering if you could do an update for. In wp-invoice-options.php on line 9, it should be "#options" with an "s":
return $options[$option_name];//return $option[$option_name];
This bug is creating an issue with notifications where the options set do not override the default information.
In wp-invoice-options.php on line 499, I found a bug that causes the tax rate to prefix a zero every time the settings form is sent. I added a sprintf function to add a leading zero if needed, instead of concatenating a string.
$plugin_options['invoice_payment_tax'] = sprintf("%02s", $plugin_options['invoice_payment_tax']); //'0'.$plugin_options['invoice_payment_tax'];
I've taken an interest in this project as it is the best (simple) invoicing plugin that I've found even in its beta form. If you are looking for collaborators on this project, I wouldn't mind contributing. Let me know if that is something you would be interested in. Thanks.