It turns out you are selling insurance.

I read an interesting thing about project-based billing today, and it goes like this:

• When you do fixed-bid projects where the scope is not clearly-defined (which is the type of work I do almost every time), you are taking on risk for another party.
• There’s a word for taking on risk for another party: Insurance.
• And there’s a word for what people pay for insurance: A premium.
  (source: Jonathan Stark)

So, if you are in the business of fixed-bid projects, make sure you are charging a premium for that insurance you are providing!