The order processing system begins immediately after a customer has placed their order. As mentioned, this is dependent on the payment processing service, but in the Google Checkout world the next step is authorization. During this phase, Google performs some anti-fraud checks on the customer's payment and then verifies that the payment method is good for the complete amount of the order.
If the customer's payment passes these safety checks, Google changes the payment status to Ready to Charge. This means the merchant can now log in to the Checkout service and charge the order. By default, this is a manual process for the merchant that requires logging into Google Checkout.
Once the merchant has charged the order, they must begin the fulfillment process, which includes picking the product from their shelf or warehouse, preparing it for shipping, packing, and sending. Once these steps are completed, the merchant can log in to Google Checkout and change the payment status to Shipped. These manual steps, charging and shipping, are what the order processing system seeks to automate, or at the very least, integrate into the tools that run our e-commerce websites.
There are two Google Checkout APIs to help with this task: the Notification API and the Order Processing API. The Notification service simply provides a means for Google Checkout to notify your application when new orders arrive, pass risk checks, or when the order is charged. This is a one-way API, your application cannot communicate with Checkout, it only receives updates.
The Order Processing API is used to initiate actions with the Checkout service. These actions include not only charging and shipping, but also refunds and cancellations. You can also use this API to manage the status of individual items in an order. This is useful for situations where items will be fulfilled or shipped separately. You can also use this API to add shipping tracking numbers to orders or partial orders, and allow your application to send messages to your buyers automatically.
These two APIs are very powerful and have myriad uses depending on the needs of your business. Not all features of the API must be implemented, so solutions can be tailored directly to the application needs.
Other payments services, such as Amazon FPS and PayPal, support a similar set of tools. For simplicity, our discussion here will cover Google Checkout, but the basic flow is similar to other vendors. The support for programmable APIs and callbacks varies from vendor to vendor, so some features of the Checkout API may not be available in FPS or vice-versa.