I have tipped on occasion for a job well done or expedited immediately.
How is this any different than getting a gift card for being the employee of the month from your manager?

Small shops have few perks.
Bringing in burritos at lunch time brings good will capital in the future.
Providing a box for free for shipping deserves a return favor.
Giving me a free discarded seat post for my MTB was greatly appreciated.

I imagine folks that work as mechs get a rash of $#!^ undeservingly from irate customers, I would rather be the guy that is seen as kind and appreciative.