Fundraising tech/IPN listener

Overview
Many payment processors send realtime updates on the state of payments to a listener endpoint on the merchant's servers. We refer to these messages as Instant Payment Notifications (IPNs), following PayPal's terminology. For some of our providers the IPNs are an essential part of the normal payments flow while for others they serve as a second opportunity to capture information about donations in case of problems sending the information from payments-wiki.

Adyen
Adyen sends us IPNs on card payment authorization and capture as well as when a new settlement report is available. Their authorization IPNs are the only way we receive CVC and AVS results, so we wait for the IPN before deciding whether to capture the payment.

Amazon
TODO

PayPal
TODO