We always get payment upfront for custom orders, including shipping, unless they plan to pick it up. We've never had anyone ask to pay later or only pay a deposit.
We bring 2-part order forms to shows, on which we list all the details, including all their contact info, so we both have a copy of everything agreed to. Then we email or call, whichever they requested, when the order is done.
That being said, if they just wanted an item that we normal have but had sold out of we would be more flexible. It's still better if they want to pay upfront, then you know you have the sale, but we would certainty offer to write/call when we had the piece available without payment.