I really like USC because of the quantity of items you can find and the efficacy to deliver them in such a short time. However, the fact that you cannot modify an order once you make it was a really big problem for me this time because I got two pairs of shoes with the wrong number and I tried to change them 10 minutes after I ordered them and it couldn't be changed.
Maybe you could just let like a margin of time to make some changes, it would be nice just in case someone made a mistake in their order like me.