I don't buy the whole it's customary to tip a service provider argument. It's a little different in our situation. I can see if I got electro maybe a couple times a year, then a tip may be in order. I see a hair stylist or a nail tech a few times a year, that sounds reasonable. But, I've spent close to $15K on electro. If I add in a tip, that's a few thousand more. And really, you don't always know what kind of a job you are getting because the pitted scarring might take a while before it's evident. And you can't always tell how accurate they are either. So, you could be potentially tipping someone who is not doing you much of a favor.
I have nothing against my zap ladies, but they is expensive. I've never been guilted by any of them that I should be tipping. Referrals, on the other hand, I've handed out plenty of those, so in a way I've given a little extra.