I've implemented the TopSelling Report in the GUI. It's not working for
some dates, which is weird. The date format required is "dd-mm-yy". 

I am not sure if I can finish online purchase. If anyone can spend time
making the query work so there's a functional result set, that would be
amazing.
complete online purchase now fills in an auto-generated date 10 days
from the purchase as the expecteddate