Do any of the independent A&P's here use a formal work order?
Heh. I obviously can't speak for anyone else, but I sure don't. While cars (and moreso, TC'd aircraft) are fairly well defined, with maintenance manuals, etc., EVERY DANG E-AB AIRCRAFT IS UNIQUE. So depending upon the job, I may give a written ESTIMATE, and I make it very clear that what I say is an estimate, NOT a quote, and that depending upon what nonsense I find when I start digging, the #'s may go up.
Mostly, estimates will be used for well defined work, like a Pre-Buy examination or condition inspection, particularly with travel, but with larger, poorly defined jobs (I'm in the beginning of doing a wiring completion on a COZY MKIV, and I handwaved 100 hours at the owner, but as I start, I find that I have had to rewire most of the engine compartment, and we'll ahve to spend a couple of days repairing holes that the previous owner had drilled in the spar cap for wiring pass-throughs. Yep) I just give a range of expected times. So this MKIV may end up being 150 hours or so, by the time we're through, or more, depending upon what I find as I move forward (geographically) in the plane. For something like a wheel pants install, I'll tell people a small range - say 20 - 25 hours of labor, and can usually hit it pretty closely.
For almost all of what _I_ do, a formal work order would be obsolete about 10 minutes after the owner agreed to it. All of my customers understand that they have unique planes and only one of them used to be pissy about how long things took and how much I charged. So I started giving him invoices that basically said:
"Here's what I'd charge a normal customer, given how much work I did, but YOU can pay me whatever you want and think is reasonable, as long as you cover my out of pocket expenses".
The complaints stopped after that, and he pays what the invoice says.
In ~370 chargeable jobs over the past 9 - 10 years, I've only had one guy try to stiff me on the bill, and his wife convinced him that he was an idiot and didn't understand what a Pre-Buy was for, if he thought that he shouldn't have to pay if the results weren't what he wanted, so he paid up.
Maybe I've been very lucky, but I work on my word and a handshake.