So here is a point of view of some one who takes commissions. For me If I know I have some time to get the prop made I will take half of the payment in advance and it is because some times it can take a couple months to find/order the materials that are needed. But what really annoys me is when people assume that you are in the same country as the person you are commissioning. I made a very large prop for a girl and had no idea what the shipping was till I was done and when I told her the cost of shipping she decided she didn't want to pay for it. So I'm stuck with a large prop in my work shop because she assumed we were both in the u.s.a. She is also wanting a refund but I did use all the money to buy the materials to make it so it is gone. So here are some tips for ppl that want to commission large props. 1. Make sure you are in the same country. It is not the job of the prop maker to check that you know where they are based out if. 2. Once a prop is made don't expect to get a refund on it. It takes time and skill to make a good prop and that is also what you are paying for. Unless you have a arrangement with the prop/costume maker to get a refund once the item is done.
Sorry I went on a bit of a rant there.