I would agree regarding tracking your product usage -- it can help you to spot any areas in your process that may need further evaluation and maybe help you determine if you are using a product that isn't as effective for the job at hand as an available alternative. Not to mention it helps you keep track of inventory and know when to reorder so you're never accidentally running out of something in the middle of a job.
However, once you've got a procedure down to a science and can ballpark the average amount of product you use per job (to determine your costs) it becomes a bit less important to track every single detail you do.
As far as documentation to be provided to the customer, it really depends on the customer and the type of job you're doing for them how in-depth you need to get on the invoice. I think for most people and their daily drivers, just a basic listing of the services rendered is good enough. For a car enthusiast's weekend toy or a collector's show cars, something more in-depth may be appropriate.
In fact, I'm working on adjusting a lot of things about my business and one of the ideas I have been kicking around is offering an "Enhanced Documentation Package" as an up-sell for customers. This would include a full detailed overview of the vehicle and all major components/systems, a detailed map of paint thickness readings across all surfaces, photo/video documentation of the vehicle and/or the detailing process, and other pertinent information presented in a binder/folder along with a CD or DVD containing multimedia content.
This would of course take some time to put together (with a template it shouldn't be too big of a killer though) and I would have to charge accordingly, but especially for an enthusiast or collector it would serve as an enlightening "snapshot in time" of the vehicle's condition to build pedigree. That is helpful at judged show events and it could also come in handy when selling/auctioning the vehicle since potential buyers would be getting full disclosure about what they'd be investing in.
Just throwing that out there, anyway.