LOL! I'd feel more comfortable having them estimate it for time and materials. That is, if I knew and trusted their work. As you point out, a "flat rate" or fixed price, is a gamble for both sides. But the yard deals the cards, so they're more likely to win.

On reading your lower post, I agree, I'd have them do it. Changing through hulls with a clock ticking sounds like some kind of warped tv show.