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If you've just changed your mind and there's no reason for you to be unhappy with the product itself (they have followed your instructions for the customisation and haven't misled you as to what it will look like) then you're stuck. They have no obligation to accept the return.
If the item isn't as you expected and they've done something wrong then you may have more luck. Ultimately they've given you what you asked for and they're now unable to resell it as they've customised it.
Depends why you're wanting to return it. Have you changed your mind, or is it faulty?