We apologise for the inconvenience caused and the late reply.
In such cases, the best way to debug this is to check the debug log in Magento.
You can get the file from this path :
Request will be in /srv/users/serverpilot/apps/onlyhearts-magento/public/var/log/stampsrequest.log file
and Response will be in /srv/users/serverpilot/apps/onlyhearts-magento/public/var/log/stampsresponse.log file.
If you can send us these files, we would be able to understand exactly what problem is causing this. Do let us know if you have any query.