I don't charge anything extra for the design, even if I have to buy it. I figure that if one person wants one, then others will want one. I almost always make two because it doesn't take much longer, and it is insurance against breakage. If all goes well, the extras get listed in my shop. Last year my best seller came from a custom order.
I might charge extra for a personalized item, but so far, all of the personalized things I have done are quite simple and only took a few minutes.
Admittedly, my selling experience is somewhat limited.