Never heard of that before! Maybe you could make a few donations of dog food?? Maybe the tips are for the workers? It could be their machine is a salvaged one from an old restaurant? I’d ask them what are the tips for? If its just for the kennel then I’d say no, as you have stated it is rather expensive already, and surely the fees should cover any outgoings the kennel has. If its for the staff, who take care of your dog or a charity then maybe, I’d consider a tip, depending on how happy my dog is when I drop it off and pick it up, if its unhappy its unlikely I’d be using them again so a tiop would be out of the question anyway.