I always tell mechanics and other people doing work for me that if it is going to be a penny over the estimate they need to call me and let me make the decision to proceed, that gives me a chance to control the cost. I have had experience on the other side of this equation where you end up looking at a bill for twice the amount and you're like ... what the hell did you do ? And it turns out they did a bunch of stuff you never agreed to up front, that you would have done yourself, or that you were paying someone else to do, etc.
Playing devil's advocate, how do you know the customer wasn't going to put the sails on the boat themselves ?
If you don't limit the amount to whatever the estimate was, how can you have any hope of controlling your costs ? It NEVER comes out to be less than what the estimate was.
Too many mechanics especially seem to think that if you drop something off to have work done on it that you've just opened your wallet up so they can help themselves ...
Edit - I have to add this, this kind of stuff is why I like to do everything myself and only farm out exactly what I want done. I refuse to take vague drawings to a welder and say hey can you build me something like this, I just don't do it anymore, I have learned my lessons the hard way. If I want something welded I take precise drawings and I say I want you to tack weld it here, here, here, and there, then I want you to flip it over and cut this here, tack weld there, and sand the edges smooth, now what are you going to charge me for that - fixed, not time and materials. That way there is no confusion.