There are two moments a shop can ask you for approval, and both arrive as a request on the booking in the app with the mechanic’s price, a breakdown, notes and any photos of what they found. Only one request can be open at a time, and the shop can withdraw one it sent by mistake.
Before the job: the inspection estimate
After inspecting the car, the shop confirms the price. If it lands within what you approved when you booked, it is confirmed automatically. If it is above, you get a request. Approve it and the work goes ahead at the new figure, which becomes the ceiling. Decline it and the shop does not start the work at that price; what happens to the $20 hold then depends on how the booking is closed. A cancellation made by the shop releases it in full; if you cancel, the windows shown on the booking apply. Leave the request unanswered for 24 hours and it is treated as declined, and the $20 deposit is kept to pay the shop for the inspection it did.
Mid-job: added work
If the mechanic finds something else once the work is under way, the shop sends a request listing the added lines. Approve it and the lines are added to the job and the ceiling rises to match. Decline it and the added lines are removed from the booking; they are never charged. If a mid-job request goes unanswered for 24 hours, the job simply continues at the last price you approved and the added work is not charged.
After the job
Once the job is done, the amount charged cannot exceed the ceiling you last approved. If something on the final receipt looks wrong, you have 14 days from the final charge to open a dispute.