A great teaching tool for girls is to have one girl track the inventory as you sell it. I generally rotate my girls every 10 minutes. The positions I have are greeter (for when they are entering the store just to say hello), asker (for when they are leaving the store, asking Would you like to Support Girl Scouts), money (make change), and inventory (mark off sold boxes and restock table as needed).
Here is a tally sheet a parent helper of mine made awhile back. I love it and use it often. Be careful of changing anything but the numbers across the top. They are set to automatically change the rest of the spreadsheet to the right colors/length of column and so forth.