I know tattoo artists who are super happy to receive a batch of home made cookies as a tip.

But if you are doing something that demands a flat rate based on time tipping as a custom seems pointless. Just charge more.

Plumbers and locksmiths charge flat rates per time. And often for travel time. Why can't that work for everything else. Prices are what they are up front.

You aren't socially expected to tip a person who keeps your basement from flooding but you are expected to tip others?