We don't really talk about rates here, too many places with different economies.
Basically, Flat Rate, just means that someone took the time to add up the ingredients for the job and posted the final figure.
It saves a lot of time, in that you figure the price once, no matter how many times you do the job.
For instance, if I do the same repetitive job over and over again, I don't waste time adding the time, and parts each time. I just get er done, and slap the number on it.
It also means that nobody is wasting time with a stop watch seeing how long you take bringing your tools in from the truck.