Dear Mr McNamara,

It was a great relief when your bill arrived and turned out to be well within the figure you had mentioned as a 'possible'.

I'm sure that, although part of the reasons was your clever solution, the rest was the sheer solid work you put into the job.
In fact, I'm sure you've underestimated the number of hours you put in.
So, in appreciation, I'm adding a small bonus.
It was a pleasure to have you and I will have no hesitation whatever recommending your firm.

Yours sincerely,

Mr P. Okehampton, Devon.