I had a guy do some great work for me. I tipped him a 20. He said no thanx but I insisted. Urns out he ends up being manager one day. I take him my stuff he gives me good prices but I still tip him. He always says no thanx but I insist.
I took my rear tandem wheel in one day to be overhauled. He did it in front of me. 20 minutes and $14 bux. I tipped him a $20. Every once in a while, he'll make an adjustment for me for nothing on the spot while other walk ins need to leave their bikes. When I started building my own wheels,he'd even take the time to check the tension for me. Free! I'd ask how much but he'll never take a tip for something like that.
I had him replace the V-Bakes on my tandem and had it back the next day with a discount on the parts!