PF's got it. Fast, clean, with as little breast beating as possible. If you need to, tell them that you appreciate them expediting the offer, but you situation has changed and that you are no longer in a position to accept it.

I don't know your situation, but most companies I do know of consider the price of flying out candidates (etc. etc.) just the cost of doing business. Sometimes they win, sometimes they lose.

But avoiding the decision is going to hurt them more in the long term. If you needed more time to MAKE your decision, you'd be well within your rights to ask for it. But if it's already made, it's time to let these folks off the hook and find themselves someone else to do the job.

Just _be_ professional about it, and it can't reflect badly on you.