I post a photo of whatever it is that the customer is purchasing, and put a very thorough description of what they're buying, what color, how many, when the delivery date is expected to be, and any other variables so that it's all in writing. Then I charge half down, with the other half to be paid in full the day before shipment is expected. I just do the first custom listing for half the total amount, and then do another custom listing for the other half, once the item is ready, stating that it's the final payment and that shipment will be the next business day, and the date that day is. I also put in how I'm shipping (USPS, UPS, whatever), if insurance is provided and who pays for it, and if there is a shipping upgrade, which I usually provide as a complimentary service for large custom orders. You can't put in too many details, believe me!
Crochetgal has a great thought there, though, I like that about the "services" and "goods" distinction. I never thought of that!
It also never occurred to me that someone would not follow through on a commitment like a custom order, I need to give that some thought so I'll have a policy in place just in case that happens. Thanks, ladies, you never fail to inspire me!