I understand, but pex fittings are between $10-20 each.  Yes, you should be accounting for those things on an invoice.  

But you said anchors.  I agree with anything that actually has a monetary value on it.  Wall plates, etc.  anchors and screws? no.  

Yes, there are mobile bar code scanners.  You can also use your iphone or droid. There are a few apps out there, there are also complete systems that cost thousands of dollars.