We apologise for the inconvenience caused and the late reply.
In such cases, the best way to.
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.