Payment systems are now available everywhere and everyone interacts with these systems every day. There are different types of transactions you can make, SALE, REFUND, Pre-Authorization, and VOID. SALE: means to buy something and its price will be deducted from your bank account. REFUND: this means that you will return something and wants your money back to your bank account. Pre-Authorization: means holding an amount of money from your account, e.g Hotel reservation. VOID: this means canceling the transaction, e.g if the seller entered the wrong amount. You are required to implement the SALE transaction only by simulating the card, terminal(ATM), and the server.