I'm trying to put together a schedule of quoteable costs, i.e. flat rate book and have a couple of questions on pricing structure.

Replacing a service with a 200 amp lends itself well to flat rate but sometimes we'll do a call of just replacing a GFCI receptacle. The job may take 20 minutes from the time you hit the customers door and the GFCI cost is around $10.00 and with markup call it $15.00. If your cost to do business requires a charge of $80.00/hour then the labor for this 20 minutes is $26.66. You can't charge the customer $41.66 to make the replacement and stay in business. Now $41.66 may be a good flat rate price for the next one but if you only have one to do it's not worth the trip. How does flat rate take care of the job that bills out for less than, say, $60.00?
Would you have a show up cost?, or a minimum billing?, or something else to 'pad' the cost of the trip?