Never ever ever ever pay up front for a custom knife. I've had multiple knives (of my own design even) made by reputable makers and they never asked for a penny up front. I expect them to contact me right before they start work and ask me if I'm still good with everything, and then when the knife is done, I pay and they ship.
I can understand how that could get away from you as a maker, too. If some guy paid me 3 years ago for a knife and that money is long gone, I might have trouble keeping the lights on and buying materials today. And even when I finish the knife, then I have to ship it out and I get nothing since it was paid for years ago. The maker would have to meticulously manage that money when he could be concentrating on making knives instead.
Just don't do it. If I have to pay up front, that's a knife I just won't have.