This module does provide Auto-Login Links for your WHMCS mails. You can modify your existing links by just adding "&{$modautologin}" to authenticate the user automatically when he opens a link to an invoice or other pages.
The authentication does work for every WHMCS page (custom module pages too) and the module has a lot of configurable options to meet every use purpose.
This can be useful for let your clients show the invoice directly or to increase the ticket feedback rate.
Overview
Content Tools