Looking good, easy to use, can use some more help instructions:
- needs write permissions in .\outputs folder
- Invoice can be viewed and emailed as PDF in the 'All invoices' menu.
- No front-end HTML view of invoice, which you may expect for clients. But it's not really necessary.
The following features would make it a killer plug-in:
- Recurring invoices, sending them automatically
- Sending email copy to a globally set cc: email adress
- Custom invoice numbering format (Ymdxx = 2013071101, 2013071102, 2013110703, ..) (http://codex.wordpress.org/Formatting_Date_and_Time)