[Company/ supplier address]
RE: [Quote number]
I was surprised to see from your invoice [invoice number] dated [date] that the cost of [describe work carried our] is [£……]. This is [£……] more than your original quote of [£……] on [date], which set out the fixed price for the work, which was the basis on which I entered into a contract with you.
Though you claim the increase is due to [reasons given], this has no bearing on our contract. There was no agreement between us at the time I commissioned you to carry out the work that I should bear these extra costs.
We agreed an exact price for the work and therefore my contractual obligation is simply to the contracted price shown in your quotation. I therefore will be paying the amount of the original quotation, in full and final settlement.