TLDR:  post IS successful.  PayTrace, by design, returns a 400 error, which sets off exceptions in httpresponse.  Solution: catch the exception and then continue deserializing your response.

I coded a few weeks ago a .NET post to the PayTrace API which helps me demo and test payment by credit card using client side encryption.  The process more or less went like this:

• Create demo account as a merchant on Paytrace
• Download PEM key
• On submit of form with credit card information, an imported PayTraceJS library encrypts the card number and csc code
• Use the demo account’s username and password to submit a request for a token
• Submit transaction (which includes encrypted info as well as other required fields) using token and await response

A successful http response returns a status code of 200.  I read it via stream, deserialize it using json into my CardResponse object (both successful and failure responses have the same design).  Everything went great until I began testing rejected cards.