It’s not unusual or unreasonable for people to want something unique. We are not all the same and that’s a good thing.
When a customer asks for something that varies in some way from the product you are offering, it’s good business to attempt to satisfy their wants. As long as the variation can be accommodated without having to retool your entire production line, a happy customer can be in easy reach.
But a problem arises when the alterations the customer wants are completely out of line with the existing product. The customer wants it made out of a different wood. And longer. Oh, and also, can you make the whatever this way instead of that way? And before long, we are talking about a completely different item, one that can’t possibly be made with the existing setup. Now we are talking about a one-off piece.
That might not be a problem if the customer is willing to pay. But, often, the customer wants these modifications for the same price as the production item.
You could say something like, “How much do you think Apple would charge if you asked for a phone a half inch bigger?” But this often only produces a blank stare.