There are a few important points here. We have a standard contract that we use for each job (particularly retro-fits) that explains are billing procedure in detail. We also have an equipment list generated by computer at design for the installer to follow so that nothing is missed. Lastly, I know it is a pain to document hours and detail everything, but it does give the customer more confidence that they are getting value for their money and this will be important for refferals, even if they don't read the itemized list. Carry on.