HTTP response codes are used to indicate the success or failure of an API request. In general, 200 indicates success. 401 indicates an authentication error, 500 indicates transaction errors given the information provided, and 400 indicates an error while updating a merchant order ID or when updating a cart.

curl -u <api-key>:<secret-key>

HTTP/1.1 500 Internal Server Error
Server: nginx/1.8.0
Date: Fri, 14 Oct 2016 15:30:46 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 82
Connection: keep-alive
{"description":"Transaction ID was invalid","error":true,"type":"INTERNAL_ERROR"}