This is the part I was thinking could be similar to istockphoto's system:
Prepaid - Callers can direct senders to the appropriate Amazon FPS co-branded user interface to purchase a prepaid balance and obtain a multi-use token for the prepaid balance. This multi-use token can be used to track the sender's usage of the prepaid balance. Amazon FPS provides GetPrepaidBalance API to obtain the outstanding prepaid balance at any time.
Higher on the page, it refers to using this in connection with micropayments, but it doesn't seem to be limited to that here.