Here's a "do you" question:
Do you itemized your invoices? Do you give materials lists down to the wirenut, or just a grand total called "materials"?
Do you give times in and out, dates, and price per hour for the labor, or just a grand total called "labor"?
Do you add a markup on materials or an overhead-and-profit percentage on the whole works, or is it hidden in the labor price?
How much information do you give your customers, and how much do you hold back?