Invoice Template Modifications

Check this: http://xlinesoft.com/invoice

There isn't much on there in terms of documentation. Just a few FAQ's.. Do you have anything in more depth?

Well, Invoice template is a PHPRunner project and there is nothing special about it. Once you have created a new project using Invoice template you can see how everything is implemented right in PHPRunner. More in depth info is PHPRunner manual: http://xlinesoft.com/phprunner/docs/

Okay, thank you. One last question: the main feature I want from this template is the item lines (the JS section that allows the user to add an item, then automatically moves down the line and adds another line to add an item). I looked at the code, but was not able to replicate it and add it to a current system that I had developed. {$displayDetailTable_invdetails} is the variable I am talking about.
Can you provide me with a breakdown or documentation on how to add it please?

I'm afraid there is no such functionality as "automatically moves down the line and adds another line to add an item". It doesn't happen automatically, you need to click 'Add item' button.
{$displayDetailTable_invdetails} - is child table snippet. In your project you need to link two tables as master-details and turn of 'Display child table on Add/Edit/View pages' option.