I’m in the service industry as well, and I typically tell them tipping is not required when they offer, but that’s a case by case to be honest. I also tell them that the best form of tipping is references, word of mouth, if they have friends/family that may need my services. I don’t work for anyone and no-one works for me, I’m a solo operation, and I charge and pay myself sufficiently enough that I can live on what I make. That being said, one of my clients gave me some ghost peppers a month or so back as a tip (tasty and hot lol, everyone was too chicken to try them so I ate them all myself), others offers beer/shots, sandwiches, homebaked goods (around holidays) but mostly it’s cash.