Contingencies - All repairs have uncertainties. You do not always know what the progress of repairing will reveal. You can agree with the client that any work required in addition to the initial agreement will be notified for the client to decide whether to proceed or not. However, you can take on the risk. This is what the contingency is for. You need to build allowance for these unforeseen developments. A 10% to 20% of the total costs addition to the price is sensible if you are taking the risk.
This may all sound like it is too much trouble for a simple repair. Yes, it does take a bit of consideration to start with. But once you have established the basic labour, travel, overhead and profit levels, the rest is pretty straight forward. You will have an idea of how long it takes to do the work, to travel, the glass costs, etc., and the profit level. You only need to multiply by the rates you have established to give you the price. I should warn you - it will be much higher than you initially thought.