I was getting the following error when I edit an invoice.
403 Permission Denied
You do not have permission for this request /wp-admin/admin.php?page=wpi_page_manage_invoice&wpi[existing_invoice][invoice_id]=100&action=edit
I use the Bullet Proof Security plugin, and I discovered the 403 error is as a result of the full query string when editing an invoice containing something in it that is being blocked by one of the BPS security filters in the root .htaccess file.
There is an EASY fix posted on the BPS author's website: http://www.ait-pro.com/aitpro-blog/2252/bulletproof-security-plugin-support/checking-plugin-compatibility-with-bps-plugin-testing-to-do-list/#wpinvoice
Just wanted to share the info with others who might also be struggling with this issue..