I agree with most everything that's been said. Be honest and make sure they know what they're getting. Do it at cost or close to it if you want to hedge your bets. You are probably your own worst critic so it'll prolly be really good. Don't do it if you are really uncomfortable though as you should not sell/serve something you doubt.
Quick story on this though: Last year I did one of the largest cooke ever for me. Twenty-eight butts for a neighborhood fundraiser, and it was using a cooker I had never seen before the day of the event. Everything went weird: The rub was too solid to use, I only had Kingsford for smoke, and the temp spiked up to 350 right at the end, AFTER I probed one of the buts at 190, but rather than take them off immdediately, some one decided to wait until the cooker cooled down some. I was really nervous, but it turned out to be really, really good. I got several compliments, and I even thought it was worth eating myself (own worst critic and all that). Turned out they pulled and sold every last butt, and it was the first year ever that they did that. I was happy and impressed. Not saying that that always happens, but it can.
Finally, a suggestion that I haven't seen before on this thread: Why not do a Chuck Roll? It's basically the Boston butt of beef and IMHO is about as forgiving. Since your client's client doesn't eat pork, this should be okay with him, and survive the ride to wherever. If you haven't done one, you might not want a for profit gig to be your first, but I think it's a great alternative. Only bad things are the cost (~$2/lb), and the fact that they take for ever to cook (~24 hours). They are boneless though and have less loss than pork butts.
dmp