We do fixed bid all of the time with a guaranteed maximum, as I'm sure you've all seen and commented on here.

We rely on the honor of our customers and they rely on ours. If little unexpected things pop up, we cover them at no charge. If something big that nobody expected comes up, then we work out a fair resolution.

We've never had a problem.

PS. If that's almost done, what did you contract for them to do? How much are they charging you?