The payment service, as the name suggests, will be responsible for collecting payment. It can offer a different way for the user to pay for his purchase, such as a bank transfer, credit card, cash, or PayPal. We can go one step ahead here and can have a wrapper service that will actually check if the current integrated payment gateway is up or down. If the current payment gateway is down, it will use an alternative option of payment gateway, if one is available. Collecting money is the main point of business; you cannot afford to lose the user here, so you need as much fallback as you can have here.