Oh, and if you find it hard to say no (I find it very hard), one of the best things you can do is set up a snippet with the text already prearranged. Something like,
"I'm sorry you can't use this [name of item here]. Unfortunately since it has your custom text on it, I cannot resell it, so I can't offer a refund. There is a repeat buyer coupon code, though [insert coupon code here], if you want to purchase again with different customization."
Then when you get a question like that, just type
Hi, [customer's name]
and then insert the snippet. It is painless.