Good question. I have wondered what people do in this situation. We sell sandwich and snack bag sets, where they're supposed to pick sizes and colors at checkout, and I often have to email people to find out what they want because they don't do it. But in that case, if I don't hear from them, I let them know that if I still haven't heard by a certain date, I'll send an assortment. I had to do that just last week.
But we just opened another shop selling kids clothing, and I have wondered the same thing there. Because it's no biggie if they get a sandwich bag instead of a snack bag, but what about sizes of clthes or jewelry like this???