This is the part of the arrangement worth reading twice, because it is different from a shop that replaces parts.
The assessment fee buys the phone being opened, the board being taken out and washed, and somebody spending ninety minutes looking at it under a lens until they can say what has died. You pay it whether the answer is this can be brought back or this cannot.
We charge that way because the looking is the work. A shop that only charges when it succeeds is a shop that has to make every board a success, and there is only one way to do that: quote a repair, take the money, and hand back a phone that works for a fortnight.
What you also get, for the same fee, is a written note of what was found. Take it anywhere. It is what stops you paying three more shops to discover the same thing, and once or twice a year somebody has taken ours to a workshop abroad and had the board fixed there, which is a good outcome and we are pleased about it.