A POS needs to be created. A customer would come to the cashier and the cashier would take items from the cart and enter the item ids to the system. The system would ask the number of items to be purchased. The system would create a bill and add line items to it. At the end, the total should be calculated and the tax should be added. The system would ask to enter the amount of money the customer would give. The system should print the entire bill with the balance to be given back to the customer.

POS (system)

Customer

Cashier

Item
  itemId
  
LineItem
  quantity

Bill
  total
  taxAmount
  moneyGivenByCustomer
  balanceToBeGivenToCustomer