"He is in back, yes," Snape replied, stepping out from behind one of the long, overstuffed shelves that cramped the store, holding a small red bottle whose label wasn't nearly large enough to contain all the warnings it was trying to convey. "You may have to wait for just a moment. Some shops have more than paying customer at a time, you see. You will notice," he continued, gesturing helpfully to the counter, "That he's installed a bell for just that purpose."
He usually rather enjoyed his excuses to get away from the students for an afternoon, if only to pick up the ingredients a touch too explosive to send by owl - but they did carry the risk of running into equally unpleasant adults.