There's some bug when you're saving the invoice using defaul theme:
Warning: Invalid argument supplied for foreach() in /home/lapetit1/public_html/huriata.com/wp-content/plugins/invoice-king-pro/themes/default/invoice_html.php on line 46
Anyway, you're plugin is easy to understand and easy but it lacks of some report tools. I don't mind to pay for it if there could be reports for: Total sellings (and for day, week or month), unpaid invoices, and having a client's history. Would be great if all the reports are ready for being sent by email (or PDF).
BTW, you're templates are in good price.