That job does not take you 20 min. It takes min 60 min. You have travel time, you have the time it took for you to either order the GFI (or stop and pick it up) You have the time on the phone to set the appointment. etc.
So you would typically have a min labor time of 1 hr or 1.5 hr or 2 hrs. It's up to you.
And look at it this way, that money has to come from somewhere, so don't be afraid to charge for it.
