After installing the Shiptheory extension in Magento, if you see the error Service Temporarily Unavailable, you may need to delete a temporary lock file used by Magento while installation was taking place.

How to Fix Service Temporarily Unavailable in Magento

Connect using FTP/SFTP to your Magento web server and in the root directly you will find a maintenance.flag. Delete this file and your store should be restored.

If in doubt, contact your Magento developer or Shiptheory support.