Mechanical cash registers have made a ringing ka-ching since the late 1800s, when the bell signalled a sale and discouraged theft, and that noise survived long after electronic tills replaced the drawers. Online, creators kept the four-hit version because one ring feels like a single purchase while four in a row suggests a rush of customers. It is often paired with a simple ding sound effect for scanned items, keeping the register as the bigger payoff for money actually changing hands.