Amazon Flexible Payments Service Part 3, Co-branded Flow

In a typical website, you want your customer to have a payment link that starts the process of them paying for services/products that your site has provided to them via Amazon. This typical process end to end is called Co-Branded UI process in Amazon's FPS documentation.
There are 3 roles in the process: Sender, Caller, and Recipient. The sender is the entity sending money. The Caller is the entity calling the Amazon FPS Api. The recipient is the entity who will receives the payment. Amazon uses tokens to provide secure handle to payment instructions.
In this process 3 tokens are used a Caller token, Recipient token, and Sender token. Your web application will need to create a Caller Token, and Recipient Token.
In the Co-Branded UI process , once you customer chooses a product/service and payment amount, they get redirected to Amazon's FPS website where they log in using their Amazon account, select their payment method. It is here in this part of the process on the Amazon site where the Sender token is created. After confirming their payment method, and amount on the Amazon site, the user gets redirect back to you site. When they are redirected back to your site, Amazon passes back to your site the Sender token. Your site will need to acquire this sender token information, and then your site will need to formally request/submit a payment authorization to Amazon FPS Webservice.
Amazon can then notify you when payment was successful either via a scheduled job that you create that queries Amazon FPS, or via Instant Payment Notification which requires you to setup a Web Service that Amazon calls when payment is successful. One of the undocumented Payment Transaction Statuses that I found is "Initiated". In their WSDL file they only define: "Success", "Failure". This can cause issues if you are not expecting to receive this status of "Initiated", and are only expecting to the 2 values of "Success" or "Failure".
The best way to get comfortable with Amazon FPS in my humble opinion is to review and work thru the examples and sample applications.