A repair quoted before the back is off is a promise nobody can keep, so we do not make it.
The figure on the booking form is what that job costs on a phone that is what we expect it to be. Then we open it, look, and ring you with the real figure. Nothing is fitted until you have said yes to the real figure, and if you say no we put the phone back together, charge the bench fee only, and hand it over.
The usual reason a figure moves is a frame that was bent by a drop nobody mentioned, or a port that is full of lint rather than broken, in which case the figure moves down. Both happen most weeks.
We write the estimate on the sheet in the box, so there is never a conversation about what was said on the telephone.