In general, I'd contact Sally/Mary to get the SDK.
From what I recall (I worked on the original Cashnet RFP), you can't format the notification URI. You just give it a generic URI and Cashnet adds query string parameters for the message.
I would turn on logging or Wireshark on the web service. Are you sure nothing is getting through?
It is of note. I don't have a Cashnet account yet. I can't test it.