Every repair gets a printed ticket and a ready-by date, is ticked off on the board, and the customer is texted when it is done.
The device, the fault, the price and the ready-by date, printed for the bench and kept with the order.
Every job waiting is on a board on the till and in the back office, ticked off as it is done.
A text or a WhatsApp message the moment the repair is finished.
Take a deposit when a part has to be ordered, and the balance on collection.
The device's serial or IMEI on the sale, found again by it later.