Labor rates are around $120/hr. Service writer creates a work order, tech finds the car in the lot and drives it into the shop. Lifts the car, pulls the wheel, dunks the tire in the water tank to find the leak, dismounts the tire, patches it, remounts it, dunks it in the water tank again to check for leaks, balances the tire, puts it back on the car, pulls the car back out, service writer finalizes the invoice, cashier rings you up. For $39 that all would have to have taken place in under 20 minutes. Kinda doubt it. Patching tires isn't a profitable service.