Since IPN is asynchronous, and our website can handle a number of transactions at the same time, it seems that when IPN sends a message containing all of the transaction information and we send it back, that IPN will send back a simple message: "VERIFIED". But suppose that more than one transaction is taking place at the same time and two messages are sent back to IPN. It responds with the "VERIFIED" message. How do we know which message it's responding to?
