TONU stands for Truck Order Not Used. It applies when a shipper or broker books a truck, the carrier dispatches it to the pickup, and then the load is cancelled after the truck is already committed or on its way. Because the carrier gave up that capacity and burned time and fuel, a TONU fee compensates them for the wasted trip.
The amount is usually agreed in advance or spelled out on the rate confirmation. For a dispatcher, making sure a TONU actually gets billed matters — a last-minute cancellation still costs the fleet real money.