Not a reliable partner
Response from opentshirts.com
You are not our customer:
We don't keep anyone domain for any reason, you were OpenTshirts Hosting customer, not OpenTshirts, We are the true owner and provider of the OpenTshirts Script and we don't hold any responsibility for any third party provider your claim is invalid you may have old version of OpenTshirts hosted with any company like Godaddy or HostGator or OpenTshirts Hosting....
your relationship with any of those company is not related to us.
You should contact your previous provider and domain registrar for help.
in your case its Public Domain Registry https://publicdomainregistry.com/
Thank you for understanding.