If you use the USEM website to schedule the interview, it requires you to pay before you can schedule the visa interview. You do this buy going to a bank and they give you a receipt number which you then use on the website. This was how we were able to schedule the initial March 10th interview. The problem is, when we try to re-use the same receipt number it shows it as being used. By paying again, the other couple received a new receipt number which allowed them to schedule another appointment.