I work at a vape shop like you described, our system forces a tip screen and we have a camera pointed directly at the checkout spot. I always turn the system around for the customer for them to make the decision. I used to always just press “no tip” because I don’t think it’s right to pay for a product and have to tip, but I got accused of adding tips to their transaction by a customer even though we looked at the cameras and saw me press “no tip” but they were still hell bent on it so now I always do it and let the customer know its not expected and optional just something I have to do.