First, you will need to login to your client area and download a copy of “WHMCS Invoice4U” plugins.
From the client’s area top menu click “Services” -> “My Services“.
Open the “View Details” box by click the arrow on the right, and click “Downloads“, in the following screen click the “Download” link.
Once downloaded, unzip the file and upload the folder “invoice4u” (as is) to your WHMCS installtion.
The folder should be uploaded to the following path: “modules/addons“.
After uploaded, the full path should be: “modules/addons/invoice4u“.
Login to your WHMCS as admin user, and navigate to “Setup” -> “Addon modules“.
Find the “WHMCS Invoice4U” module, and activate it (You will receive a confirmation message).
Now go back to “WHMCS Invoice4U“, click “Configure” to add your license key.
Save your changes and your module is all set.
You will be able to access it throught the “Addons” top menu.