I once had a lawnmower that was in bad need of repair during the busy season and the mechanic was telling me six-to-eight weeks wait. When I mentioned beer, that wait time reduced to 'next week'...He said he liked Budweiser so I got him a six pack of tall-boys, put them on ice, and brought them in a cooler. Got my mower back in 2 days.
I only had one bike mechanic who I liked enough to buy beer...sometimes it was my bike and other times it was another customers' bike he needed to work on, but if it wasn't super-busy, he'd let me hang out and would teach me stuff about bike repair and we'd talk about beers, so I knew he had an idea of what beers he liked. He was as much as buddy as he was my wrench.
Maybe it was a 'tip' but I always paid whatever he charged and never thought of it as a tip --- more shared beer appreciation with a friend.