I would think though that a service call includes some initial
amount of time. Maybe the 20min/$90 the OP spoke of was for time
beyond that.

Some do. Some dont. Mine does. I charge one price to come out and
diagnosis the problem. If it takes me 3 hrs to diagnose, it looks
like you win. If I diagnose it in 5 mins, I guess I win. Its kind
of an "average". I charge what it takes to keep my doors open and
make a descent profit so I will be there the next time your
appliance needs repair.
Bubba

I had a new guy come fix the furnace. He turned a valve, talked a lot,
tapped on another valve with a crescent wrench, then charged $167 for
his time. The furnace was making heat, so I paid it, but he won't be
getting a call for all the rest of the stuff he wanted to do "for" me.