Typically a flat-rate company will not quote the customer over the phone. They will tell them they need to make a $50 service call to diagnosis the problem. Once there, they figure out what needs to be done and offer the customer a discount equal to the service call if they do it right away.
The trick is to not ever tell the customer what the hourly rate is. It is often much higher than a union shop would get for a 20-year journeyman even though the flat-rate plumber has only been on the job for a year or so.
When estimating the cost of the job the plumber uses a flat-rate list to tell him how many hours to charge. An example might be something like this:
Install lav faucet - 1.5 hour
Snake lav drain – 1 hour
Replace P-trap - .50 hours
Replace angle stops - .50 X 2 units = 1.0 hours
Replace lav supplies - .50 X 2 units = 1.0 hours
The total labor not including materials comes to 5.0 hours. Assuming the labor rate is $109.50 per hour the customer will pay $547.50 in labor plus parts even though the job only takes the plumber 1.5 hours to complete. As individual units the hours may be fair but when they are done together they do not give a discount for the fact that you are already there with your tools out.
"Somewhere a Village is Missing Twelve Idiots!" - Casey Anthony
I never lost a cent on the jobs I didn't get!