Just state your terms and working conditions upfront when the order is placed. I can see why they might have been confused here if you said you were going to show them work in process - perhaps they interpreted that to mean they would have some choice. Anyway, when I had my shop, I would do custom samples but they were charged out as a regular order - never given away for free, and always done well in advance of production of the actual order. Otherwise, I wouldn't allow any choice. I stated these terms clearly in written contracts for large orders.